to adopt strict shareable rule.

* (basically) shareable objects only refer shareable objects
* (exception) shareable objects can refere unshareable objects
  but should not leak reference to unshareable objects to Ruby world

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.

Symbol last_id was changed to next_id, but it remained to be set to
tNEXT_ID - 1 initially, causing the initial static symbol to overlap with
the last built-in symbol in id.def.

Since static symbols allocate memory, we should deallocate them at shutdown
to prevent memory leaks from being reported with RUBY_FREE_AT_EXIT.

We don't need the VM lock if we make static symbol IDs atomic.

It has been aliased as ID_INTERNAL for a long time and that alias
is much more descriptive.