* `RB_OBJ_SET_SHAREABLE(obj)` makes obj shareable.
All of reachable objects from `obj` should be shareable.
* `RB_OBJ_SET_FROZEN_SHAREABLE(obj)` same as above
but freeze `obj` before making it shareable.
Also `rb_gc_verify_shareable(obj)` is introduced to check
the `obj` does not violate shareable rule (an shareable object
only refers shareable objects) strictly.
The rule has some exceptions (some shareable objects can refer to
unshareable objects, such as a Ractor object (which is a shareable
object) can refer to the Ractor local objects.
To handle such case, `check_shareable` flag is also introduced.
`STRICT_VERIFY_SHAREABLE` macro is also introduced to verify
the strict shareable rule at `SET_SHAREABLE`.