[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>

Add a new macro BASE_SLOT_SIZE that determines the slot size.

For Variable Width Allocation (compiled with USE_RVARGC=1), all slot
sizes are powers-of-2 multiples of BASE_SLOT_SIZE.

For USE_RVARGC=0, BASE_SLOT_SIZE is set to sizeof(RVALUE).

`@s1.set_len(3)` will raise so the `assert_equal` will never be ran.

A short (2 bytes) will cause unaligned struct accesses when strings are
used as a buffer to directly store binary data.

Already these had been announced to be removed in 3.2.

This commit adds support for embedded strings with variable capacity and
uses Variable Width Allocation to allocate strings.

If called with an autoloaded encoding that was not yet
initialized, `rb_enc_interned_str` would crash with
a NULL pointer exception.

See: https://github.com/ruby/ruby/pull/4119#issuecomment-800189841