This is preparation to getting rid of `T_OBJECT` transitions.
By first only replicating the information it's easier to ensure
consistency.

Now that we have the `heap_index` in shape flags we no longer
need `T_OBJECT` shapes.

Now that there no longer multiple shape roots, all we need to do
when moving an object from one slot to the other is to update the
`heap_index` part of the shape_id.

Since this never need to create a shape transition, it will always
work and never result in a complex shape.

Now that `rb_shape_traverse_from_new_root` has been eliminated there's
no longer any reason to pin these objects, because we no longer
need to traverse shapes downward during compaction.