The `Data` mentioned here was the old `T_DATA` class, not the current
extended `Struct`.

The `p->field = rb_gc_location(p->field)` isn't ideal because it means all
references are rewritten on compaction, regardless of whether the referenced
object has moved. This isn't good for caches nor for Copy-on-Write.

`rb_gc_mark_and_move` avoid needless writes, and most of the time allow to
have a single function for both marking and updating references.

This write barrier occurred before the entry was added to the table, so
if GC occurred when inserting into the table, the write could be missed.

As well as `RB_OBJ_SHAPE_ID` -> `rb_obj_shape_id`
and `RSHAPE` is now a simple alias for `rb_shape_lookup`.

I tried to turn all these into `static inline` but I'm having
trouble with `RUBY_EXTERN rb_shape_tree_t *rb_shape_tree_ptr;`
not being exposed as I'd expect.

And `rb_shape_get_shape` -> `RB_OBJ_SHAPE`.

As well as `rb_shape_edges_count` and `rb_shape_memsize`.