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 arbitrary data pointer 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.

This PR also provides example implementations for dir_data and
enumerator

TODO (Post merge)

• Implement for more types