tcFamTyPats now must take information about the instantiation of any
class variables, when checking the instance of an associated type.

Getting this to work out required some unexpected refactoring in
TcDeriv. TcDeriv needs to look at class instances because of the
possibility of associated datatypes with `deriving` specs. TcDeriv
worked over the user-specified instances. But any data family instances
were already processed, and TcDeriv had no way of finding the rep
tycons. Indeed, TcDeriv *re-type-checked* any data family instances
in an attempt to rediscover what GHC already knew. So, this commit
introduces better tracking of compiled data families between TcInstDcls
and TcDeriv to streamline all of this.