It is exported only for the extension library to test, but the method
is no longer used since 29e31e72fb5a14194a78ec974c4ba56c33ad8d45.

In such case the pointer need to be casted.

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.

We want to always use time_set_timew, as timew is 64-bit even on 32-bit
platforms so we need to be careful to both write that size, but still
trigger write barriers if we end up with a heap object.