Any and Any::get_type_id should not be stable

[reem]

The Any trait is not in release condition at all, considering that is still uses slow virtual calls that could be replaced with an associated static in the near future but stabilizing with get_type_id would mean that it could not be removed.

Normally, this isn't an issue, but since Any is almost exclusively used through trait objects this blocks possible optimizations to layout that could allow Box<Any> to be represented as *mut (), TypeId instead of containing a full vtable.

[huonw]

It would need to be (*mut (), *const (fn(*mut u8), TypeId)), where the fn is the destructor (btw).