[Bug #20025] Check if upper/lower before fallback to case-folding

	[Bug #20322] Fix rb_enc_interned_str_cstr null encoding

	The documentation for `rb_enc_interned_str_cstr` notes that `enc` can be
	a null pointer, but this currently causes a segmentation fault when
	trying to autoload the encoding. This commit fixes the issue by checking
	for NULL before calling `rb_enc_autoload`.

	[Bug #19902] Update the coderange regarding the changed region

	---
	 ext/-test-/string/set_len.c       | 10 ++++++++++
	 string.c                          | 27 +++++++++++++++++++++++++++
	 test/-ext-/string/test_set_len.rb | 29 +++++++++++++++++++++++++++++
	 3 files changed, 66 insertions(+)

* Remove `require 'io/wait'` as it's part of core now.

* Update ruby specs using version gates.

* Add note about why it's conditional.

This commit adds a `capacity` field to shapes, and adds shape
transitions whenever an object's capacity changes. Objects which are
allocated out of a bigger size pool will also make a transition from the
root shape to the shape with the correct capacity for their size pool
when they are allocated.

This commit will allow us to remove numiv from objects completely, and
will also mean we can guarantee that if two objects share shapes, their
IVs are in the same positions (an embedded and extended object cannot
share shapes). This will enable us to implement ivar sets in YJIT using
object shapes.

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

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

This reverts commit 9a6803c90b817f70389cae10d60b50ad752da48f.

This reverts commit 68bc9e2e97d12f80df0d113e284864e225f771c2.