Instead `shape_id_t` higher bits contain flags, and the first one
tells whether the shape is frozen.

This has multiple benefits:
  - Can check if a shape is frozen with a single bit check instead of
    dereferencing a pointer.
  - Guarantees it is always possible to transition to frozen.
  - This allow reclaiming `FL_FREEZE` (not done yet).

The downside is you have to be careful to preserve these flags
when transitioning.

Freezing an object changes its `shape_id` This is necessary
so that `setivar` routines can use the `shape_id` as a cache key
and save on checking the frozen status every time.

However for `getivar` routines, this causes needless cache misses.
By clearing that bit we increase hit rate in codepaths that see
both frozen and mutable objects.