Optimization: Introduce CompactAnnotation

[odersky]

This one takes a type argument instead of as a tree argument. This makes mapping such annotations a lot faster and also safer. In fact, in retrospect, most annotations could be represented as CompactAnnotations.

CompactAnnotation is extended by RetainingAnnotation, which is reserved for @retains, @retainsByName and @retainsCap. We make sure that all annotations with these classes are represented as RetainingAnnotations.

For now there are no CompactAnnotations other than @RetainingAnnotations but this could be changed in the future.