Implement additional type reservations

Implement additional type reservations

 * The class names false, true and null are now reserved.
 * The code dealing with reserved class names is now decoupled
   from scalar type hint handling. It also includes self, parent,
   and static, which are class names which were already reserved
   previously.
 * Reuse existing messages for reserved class names.

Fallout: class_alias() can no longer alias self, parent and static.
However this never really worked in the first place, as the test
which was testing this shows.