Ruby

Get rid of frozen shapes.

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.