This commit generalises macro type-checking, making class constraints
more general by allowing what GHC calls FlexibleContexts and
FlexibleInstances.

Key changes:

  - A special monad for solving, TcSolveM.
    This monad keeps track of a work list and a set of
    inert (= fully processed) constraints.

  - Solving a constraint using a top-level instance may now add
    additional constraints to the work list.

  - Instances are keyed using a TrieMap, similar to GHC's RoughMap,
    which avoids traversing all instances when doing instance matching.

The solver didn't handle equalities properly, as it didn't account for
the result of unification. Now, we try to unify, and if the unifier
makes no progress we make the constraint into an inert constraint.