Fast path in Namer for methods without defaults

Either direct defaults, or directly inherited.

The avoids `addDefaultGetters` in most cases, and the expensive
duplication of trees therein.
Optimize enforcement of dependent method type restrictions

  - No need to check the result type, as dependent method types
    are now enabled unconditionally.
  - This also means we can only need to check methods with two or
    more parameter lists.