[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(+)

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>

Fixes the following issue when compiling using C99:

ext/-test-/rb_call_super_kw/rb_call_super_kw.c
ext/-test-/random/loop.c:16:39: error: extra ';' outside of a function [-Werror,-Wextra-semi]
RB_RANDOM_DEFINE_INIT_INT32_FUNC(loop);

Distinguish initialization by single word from initialization by
array.

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

As commented in include/ruby/internal/abi.h, since teeny versions of
Ruby should guarantee ABI compatibility, `RUBY_ABI_VERSION` has no role
in released versions of Ruby.