Revamped the typing system.

Implemented isAssignableFrom almost according to spec.
Refactored clients of the typing system.
Added accurate Constant computation.
Rebuilt a couple of inspections.
Removed the GoUnderlyingType tree.
A lot of other cleanups.
Also Fixes #858
Fixed getRepresentation(GoType, GoFile) for typed constants.
Marked some tests as @Ignore