Make String#{strip,lstrip}{,!} strip leading NUL bytes

	The documentation already specifies that they strip whitespace
	and defines whitespace to include null.

	This wraps the new behavior in the appropriate guards in the specs,
	but does not specify behavior for previous versions, because this
	is a bug that could be backported.

	Fixes [Bug #17467]
	---
	 spec/ruby/core/string/lstrip_spec.rb | 18 ++++++++++++------
	 spec/ruby/core/string/strip_spec.rb  | 22 ++++++++++------------
	 string.c                             |  4 ++--
	 test/ruby/test_string.rb             | 16 ++++++++++++++++
	 4 files changed, 40 insertions(+), 20 deletions(-)

Which UTF-8 char corresponds to the binary representation is
nonsense for other encodings, and just confusing.

The mapping table is generated from the ICU project:
  https://github.com/unicode-org/icu/blob/master/icu4c/source/data/mappings/ibm-720_P100-1997.ucm

Fixes bug 16233 : https://bugs.ruby-lang.org/issues/16233

This modifies the following String methods to return String instances
instead of subclass instances:

* String#*
* String#capitalize
* String#center
* String#chomp
* String#chop
* String#delete
* String#delete_prefix
* String#delete_suffix
* String#downcase
* String#dump
* String#each/#each_line
* String#gsub
* String#ljust
* String#lstrip
* String#partition
* String#reverse
* String#rjust
* String#rpartition
* String#rstrip
* String#scrub
* String#slice!
* String#slice/#[]
* String#split
* String#squeeze
* String#strip
* String#sub
* String#succ/#next
* String#swapcase
* String#tr
* String#tr_s
* String#upcase

This also fixes a bug in String#swapcase where it would return the
receiver instead of a copy of the receiver if the receiver was the
empty string.

Some string methods were left to return subclass instances:

* String#+@
* String#-@

Both of these methods will return the receiver (subclass instance)
in some cases, so it is best to keep the returned class consistent.

Fixes [#10845]