Check negative integer underflow

	Many of Oniguruma functions need valid encoding strings

This reverts commit a54c717c7a74b91a3cdf20742c355e3ea42052d1.

	[Bug #20292] Truncate embedded string to new capacity

	Fix -Wsign-compare on String#initialize
	MIME-Version: 1.0
	Content-Type: text/plain; charset=UTF-8
	Content-Transfer-Encoding: 8bit

	../string.c:1886:57: warning: comparison of integer expressions of different signedness: ‘size_t’ {aka ‘long unsigned int’} and ‘long int’ [-Wsign-compare]
	 1886 |                 if (STR_EMBED_P(str)) RUBY_ASSERT(osize <= str_embed_capa(str));
	      |                                                         ^~

	Fix coderange of invalid_encoding_string.<<(ord)

	Appending valid encoding character can change coderange from invalid to valid.
	Example: "\x95".force_encoding('sjis')<<0x5C will be a valid string "\x{955C}"
	---
	 string.c                 | 6 +++++-
	 test/ruby/test_string.rb | 3 +++
	 2 files changed, 8 insertions(+), 1 deletion(-)

	Fix memory leak in grapheme clusters

	[Bug #20150]

	String#grapheme_cluters and String#each_grapheme_cluster leaks memory
	because if the string is not UTF-8, then the created regex will not
	be freed.

	For example:

	    str = "hello world".encode(Encoding::UTF_32LE)

	    10.times do
	      1_000.times do
	        str.grapheme_clusters
	      end

	      puts `ps -o rss= -p #{$$}`
	    end

	Before:

	    26000
	    42256
	    59008
	    75792
	    92528
	    109232
	    125936
	    142672
	    159392
	    176160

	After:

	    9264
	    9504
	    9808
	    10000
	    10128
	    10224
	    10352
	    10544
	    10704
	    10896
	---
	 string.c                 | 98 +++++++++++++++++++++++++++++++-----------------
	 test/ruby/test_string.rb | 11 ++++++
	 2 files changed, 75 insertions(+), 34 deletions(-)

	[Bug #19748] Fix out-of-bound access in `String#byteindex`

	---
	 string.c                 | 17 +++++++----------
	 test/ruby/test_string.rb |  3 +++
	 2 files changed, 10 insertions(+), 10 deletions(-)

	String#bytesplice should return self

	In Feature #19314, we concluded that the return value of String#bytesplice
	should be changed from the source string to the receiver, because the source
	string is useless and confusing when extra arguments are added.

	This change should be included in Ruby 3.2.1.
	---
	 string.c                 | 4 ++--
	 test/ruby/test_string.rb | 2 +-
	 2 files changed, 3 insertions(+), 3 deletions(-)

Calling `String#scan` without a block creates an incomplete MatchData
object whose `RMATCH(match)->str` is Qfalse. Usually this object is not
leaked, but it was possible to pull it by using ObjectSpace.each_object.

This change hides the internal MatchData object by using rb_obj_hide.

Fixes [Bug #19159]

It's questionable whether we want to allow rstrip to work for strings
where the broken coderange occurs before the trailing whitespace and
not after, but this approach is probably simpler, and I don't think
users should expect string operations like rstrip to work on broken
strings.

In some cases, this changes rstrip to raise
Encoding::CompatibilityError instead of ArgumentError.  However, as
the problem is related to an encoding issue in the receiver, and due
not due to an issue with an argument, I think
Encoding::CompatibilityError is the more appropriate error.

Fixes [Bug #18931]

Co-Authored-By: Aaron Patterson <tenderlove@ruby-lang.org>