[Bug #20585]

This was changed in 36a06efdd9f0604093dccbaf96d4e2cb17874dc8 because
`String.new(1024)` would end up allocating `1025` bytes, but the problem
with this change is that the caller may be trying to right size a String.

So instead, we should just better document the behavior of `capacity:`.

Co-authored-by: Jean Boussier <jean.boussier@gmail.com>

	[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`.

If the required capacity would fit in an embded string,
returns one.

This can reduce malloc churn for code that use string buffers.

assert_separately adds --disable=gems so we don't need to add
--disable-gems when calling assert_separately.

* Unify length field for embedded and heap strings

The length field is of the same type and position in RString for both
embedded and heap allocated strings, so we can unify it.

* Remove RSTRING_EMBED_LEN

Remove !USE_RVARGC code

[Feature #19579]

The Variable Width Allocation feature was turned on by default in Ruby
3.2. Since then, we haven't received bug reports or backports to the
non-Variable Width Allocation code paths, so we assume that nobody is
using it. We also don't plan on maintaining the non-Variable Width
Allocation code, so we are going to remove it.

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>