These must not be changed once initialized.

Followup: ac123f167a364c3d7a43eca78d564e41f6dbb91e

RB_WARN_CATEGORY_ALL_BITS is exposed in a public header, so it
makes sense for it to be updated to contain all valid bits.

Instead we introduce RB_WARN_CATEGORY_DEFAULT_BITS to list the
categories that are enabled by default.

[Feature #19538]

This new `peformance` warning category is disabled by default.
It needs to be specifically enabled via `-W:performance` or `Warning[:performance] = true`

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.

When using rb_data_type_struct to wrap a C struct, that C struct can
contain VALUE references to other Ruby objects.

If this is the case then one must also define dmark and optionally
dcompact callbacks in order to allow these objects to be correctly
handled by the GC. This is suboptimal as it requires GC related logic to
be implemented by extension developers. This can be a cause of subtle
bugs when references are not marked of updated correctly inside these
callbacks.

This commit provides an alternative approach, useful in the simple case
where the C struct contains VALUE members (ie. there isn't any
conditional logic, or data structure manipulation required to traverse
these references).

In this case references can be defined using a declarative syntax
as a list of edges (or, pointers to references).

A flag can be set on the rb_data_type_struct to notify the GC that
declarative references are being used, and a list of those references
can be assigned to the dmark pointer instead of a function callback, on
the rb_data_type_struct.

Macros are also provided for simple declaration of the reference list,
and building edges.

To avoid having to also find space in the struct to define a length for
the references list, I've chosed to always terminate the references list
with RUBY_REF_END - defined as UINTPTR_MAX. My assumption is that no
single struct will ever be large enough that UINTPTR_MAX is actually a
valid reference.