New doc about encoding (#5572)

This is the beginning of an extended explication of Ruby encoding.

One of its more important jobs is to provide link targets for encoding documentation in other classes (String, File, IO, etc.). In particular, they can link to the "Encoding Options" section.

I'll have much to add to this document going forward, along with suitable adjustments in the class documentation.

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+== \Encoding
+
+=== The Basics
+
+A {character encoding}[https://en.wikipedia.org/wiki/Character_encoding],
+often shortened to _encoding_, is a mapping between:
+
+- A sequence of 8-bit bytes (each byte in the range <tt>0..255</tt>).
+- Characters in a specific character set.
+
+Some character sets contain only 1-byte characters;
+{US-ASCII}[https://en.wikipedia.org/wiki/ASCII], for example, has 256 1-byte characters.
+This string, encoded in US-ASCII, has six characters that are stored as six bytes:
+
+  s = 'Hello!'.encode('US-ASCII')  # => "Hello!"
+  s.encoding                       # => #<Encoding:US-ASCII>
+  s.bytes                          # => [72, 101, 108, 108, 111, 33]
+
+Other encodings may involve multi-byte characters.
+{UTF-8}[https://en.wikipedia.org/wiki/UTF-8], for example,
+encodes more than one million characters, encoding each in one to four bytes.
+The lowest-valued of these characters correspond to ASCII characters,
+and so are 1-byte characters:
+
+  s = 'Hello!' # => "Hello!"
+  s.bytes      # => [72, 101, 108, 108, 111, 33]
+
+Other characters, such as the Euro symbol, are multi-byte:
+
+  s = "\u20ac" # => "€"
+  s.bytes      # => [226, 130, 172]
+
+=== The \Encoding \Class
+
+==== \Encoding Objects
+
+Ruby encodings are defined by constants in class \Encoding.
+There can be only one instance of \Encoding for each of these constants.
+\Method Encoding.list returns an array of \Encoding objects (one for each constant):
+
+  Encoding.list.size        # => 103
+  Encoding.list.first.class # => Encoding
+  Encoding.list.take(3)
+  # => [#<Encoding:ASCII-8BIT>, #<Encoding:UTF-8>, #<Encoding:US-ASCII>]
+
+==== Names and Aliases
+
+\Method Encoding#name returns the name of an \Encoding:
+
+  Encoding::ASCII_8BIT.name  # => "ASCII-8BIT"
+  Encoding::WINDOWS_31J.name # => "Windows-31J"
+
+An \Encoding object has zero or more aliases;
+method Encoding#names returns an array containing the name and all aliases:
+
+  Encoding::ASCII_8BIT.names
+  # => ["ASCII-8BIT", "BINARY"]
+  Encoding::WINDOWS_31J.names
+  #=> ["Windows-31J", "CP932", "csWindows31J", "SJIS", "PCK"]
+
+\Method Encoding.aliases returns a hash of all alias/name pairs:
+
+  Encoding.aliases.size # => 71
+  Encoding.aliases.take(3)
+  # => [["BINARY", "ASCII-8BIT"], ["CP437", "IBM437"], ["CP720", "IBM720"]]
+
+\Method Encoding.name_list returns an array of all the encoding names and aliases:
+
+  Encoding.name_list.size # => 175
+  Encoding.name_list.take(3)
+  # => ["ASCII-8BIT", "UTF-8", "US-ASCII"]
+
+\Method +name_list+ returns more entries than method +list+
+because it includes both the names and their aliases.
+
+\Method Encoding.find returns the \Encoding for a given name or alias, if it exists:
+
+  Encoding.find("US-ASCII")       # => #<Encoding:US-ASCII>
+  Encoding.find("US-ASCII").class # => Encoding
+
+==== Default Encodings
+
+\Method Encoding.find, above, also returns a default \Encoding
+for each of these special names:
+
+- +external+: the default external \Encoding:
+
+    Encoding.find("external") # => #<Encoding:UTF-8>
+
+- +internal+: the default internal \Encoding (may be +nil+):
+
+    Encoding.find("internal") # => nil
+
+- +locale+: the default \Encoding for a string from the environment:
+
+    Encoding.find("locale") # => #<Encoding:UTF-8>  # Linux
+    Encoding.find("locale") # => #<Encoding:IBM437> # Windows
+
+- +filesystem+: the default \Encoding for a string from the filesystem:
+
+    Encoding.find("filesystem") # => #<Encoding:UTF-8>
+
+\Method Encoding.default_external returns the default external \Encoding:
+
+  Encoding.default_external # => #<Encoding:UTF-8>
+
+\Method Encoding.default_external= sets that value:
+
+  Encoding.default_external = 'US-ASCII' # => "US-ASCII"
+  Encoding.default_external              # => #<Encoding:US-ASCII>
+
+\Method Encoding.default_internal returns the default internal \Encoding:
+
+  Encoding.default_internal # => nil
+
+\Method Encoding.default_internal= sets the default internal \Encoding:
+
+  Encoding.default_internal = 'US-ASCII' # => "US-ASCII"
+  Encoding.default_internal              # => #<Encoding:US-ASCII>
+
+==== Compatible Encodings
+
+\Method Encoding.compatible? returns whether two given objects are encoding-compatible
+(that is, whether they can be concatenated);
+returns the \Encoding of the concatenated string, or +nil+ if incompatible:
+
+  rus = "\u{442 435 441 442}"
+  eng = 'text'
+  Encoding.compatible?(rus, eng) # => #<Encoding:UTF-8>
+
+  s0 = "\xa1\xa1".force_encoding('iso-8859-1') # => "\xA1\xA1"
+  s1 = "\xa1\xa1".force_encoding('euc-jp')     # => "\x{A1A1}"
+  Encoding.compatible?(s0, s1)                 # => nil
+
+==== \Encoding Options
+
+A number of methods in the Ruby core accept keyword arguments as encoding options.
+
+Some of the options specify or utilize a _replacement_ _string_, to be used
+in certain transcoding operations.
+A replacement string may be in any encoding that can be converted
+to the encoding of the destination string.
+
+These keyword-value pairs specify encoding options:
+
+- For an invalid byte sequence:
+
+  - <tt>:invalid: nil</tt> (default): Raise exception.
+  - <tt>:invalid: :replace</tt>: Replace each invalid byte sequence
+    with the replacement string.
+
+  Examples:
+
+    s = "\x80foo\x80"
+    s.encode('ISO-8859-3') # Raises Encoding::InvalidByteSequenceError.
+    s.encode('ISO-8859-3', invalid: :replace) # => "?foo?"
+
+- For an undefined character:
+
+  - <tt>:undef: nil</tt> (default): Raise exception.
+  - <tt>:undef: :replace</tt>: Replace each undefined character
+    with the replacement string.
+
+  Examples:
+
+    s = "\x80foo\x80"
+    "\x80".encode('UTF-8', 'ASCII-8BIT') # Raises Encoding::UndefinedConversionError.
+    s.encode('UTF-8', 'ASCII-8BIT', undef: :replace) # => "�foo�"
+
+
+- Replacement string:
+
+  - <tt>:replace: nil</tt> (default): Set replacement string to default value:
+    <tt>"\uFFFD"</tt> ("�") for a Unicode encoding, <tt>'?'</tt> otherwise.
+  - <tt>:replace: _some_string_</tt>: Set replacement string to the given +some_string+;
+    overrides +:fallback+.
+
+  Examples:
+
+    s = "\xA5foo\xA5"
+    options = {:undef => :replace, :replace => 'xyzzy'}
+    s.encode('UTF-8', 'ISO-8859-3', **options) # => "xyzzyfooxyzzy"
+
+- Replacement fallback:
+
+  One of these may be specified:
+
+  - <tt>:fallback: nil</tt> (default): No replacement fallback.
+  - <tt>:fallback: _hash_like_object_</tt>: Set replacement fallback to the given
+    +hash_like_object+; the replacement string is <tt>_hash_like_object_[X]</tt>.
+  - <tt>:fallback: _method_</tt>: Set replacement fallback to the given
+    +method+; the replacement string is <tt>_method_(X)</tt>.
+  - <tt>:fallback: _proc_</tt>: Set replacement fallback to the given
+    +proc+; the replacement string is <tt>_proc_[X]</tt>.
+
+  Examples:
+
+    s = "\u3042foo\u3043"
+
+    hash = {"\u3042" => 'xyzzy'}
+    hash.default = 'XYZZY'
+    s.encode('ASCII', fallback: h) # => "xyzzyfooXYZZY"
+
+    def (fallback = "U+%.4X").escape(x)
+      self % x.unpack("U")
+    end
+    "\u{3042}".encode("US-ASCII", fallback: fallback.method(:escape)) # => "U+3042"
+
+    proc = Proc.new {|x| x == "\u3042" ? 'xyzzy' : 'XYZZY' }
+    s.encode('ASCII', fallback: proc) # => "XYZZYfooXYZZY"
+
+- XML entities:
+
+  One of these may be specified:
+
+  - <tt>:xml: nil</tt> (default): No handling for XML entities.
+  - <tt>:xml: :text</tt>: Treat source text as XML;
+    replace each undefined character
+    with its upper-case hexdecimal numeric character reference,
+    except that:
+
+    - <tt>&</tt> is replaced with <tt>&amp;</tt>.
+    - <tt><</tt> is replaced with <tt>&lt;</tt>.
+    - <tt>></tt> is replaced with <tt>&gt;</tt>.
+
+  - <tt>:xml: :attr</tt>: Treat source text as XML attribute value;
+    replace each undefined character
+    with its upper-case hexdecimal numeric character reference,
+    except that:
+
+    - The replacement string <tt>r</tt> is double-quoted (<tt>"r"</tt>).
+    - Each embedded double-quote is replaced with <tt>&quot;</tt>.
+    - <tt>&</tt> is replaced with <tt>&amp;</tt>.
+    - <tt><</tt> is replaced with <tt>&lt;</tt>.
+    - <tt>></tt> is replaced with <tt>&gt;</tt>.
+
+  Examples:
+
+    s = 'foo"<&>"bar' + "\u3042"
+    s.encode('ASCII', xml: :text) # => "foo\"&lt;&amp;&gt;\"bar&#x3042;"
+    s.encode('ASCII', xml: :attr) # => "\"foo&quot;&lt;&amp;&gt;&quot;bar&#x3042;\""
+
+
+- Newlines:
+
+  One of these may be specified:
+
+  - <tt>:cr_newline: true</tt>: Replace each line-feed character (<tt>"\n"</tt>)
+    with a carriage-return character (<tt>"\r"</tt>).
+  - <tt>:crlf_newline: true</tt>: Replace each line-feed character (<tt>"\n"</tt>)
+    with a carriage-return/line-feed string (<tt>"\r\n"</tt>).
+  - <tt>:universal_newline: true</tt>: Replace each carriage-return/line-feed string
+    (<tt>"\r\n"</tt>) with a line-feed character (<tt>"\n"</tt>).
+
+  Examples:
+
+    s = "\n \r\n"                              # => "\n \r\n"
+    s.encode('ASCII', cr_newline: true)        # => "\r \r\r"
+    s.encode('ASCII', crlf_newline: true)      # => "\r\n \r\r\n"
+    s.encode('ASCII', universal_newline: true) # => "\n \n"