rb_str_modify_expand: clear the string coderange

	[Bug #19468]

	b0b9f7201acab05c2a3ad92c3043a1f01df3e17f errornously stopped
	clearing the coderange.

	Since `rb_str_modify` clears it, `rb_str_modify_expand`
	should too.
	---
	 string.c | 1 +
	 1 file changed, 1 insertion(+)

	Set STR_SHARED_ROOT flag on root of string

	---
	 string.c | 1 +
	 1 file changed, 1 insertion(+)

	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(-)

	Fix re-embedding of strings during compaction

	The reference updating code for strings is not re-embedding strings
	because the code is incorrectly wrapped inside of a
	`if (STR_SHARED_P(obj))` clause. Shared strings can't be re-embedded
	so this ends up being a no-op. This means that strings can be moved to a
	large size pool during compaction, but won't be re-embedded, which would
	waste the space.
	---
	 gc.c                         | 16 +++++++++-------
	 string.c                     | 12 ++++++++----
	 test/ruby/test_gc_compact.rb |  8 ++++----
	 3 files changed, 21 insertions(+), 15 deletions(-)

	[Bug #19319] Fix crash in rb_str_casemap

	The following code crashes on my machine:

	```
	GC.stress = true

	str = "testing testing testing"

	puts str.capitalize
	```

	We need to ensure that the object `buffer_anchor` remains on the stack
	so it does not get GC'd.
	---
	 string.c | 2 ++
	 1 file changed, 2 insertions(+)

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]