Use a monotonically increasing number for object_id

This changes object_id from being based on the objects location in
memory (or a nearby memory location in the case of a conflict) to be
based on an always increasing number.

This number is a Ruby Integer which allows it to overflow the size of a
pointer without issue (very unlikely to happen in real programs
especially on 64-bit, but a nice guarantee).

This changes obj_to_id_tbl and id_to_obj_tbl to both be maps of Ruby
objects to Ruby objects (previously they were Ruby object to C integer)
which simplifies updating them after compaction as we can run them
through gc_update_table_refs.

Co-authored-by: Aaron Patterson <tenderlove@ruby-lang.org>

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