fix overload fixpoint resolution

[dylex]

As of commit af25439, overload fixpoint resolution does not properly deal with mutually recursive functions. The following fails:

f () = h (Type (f ()))

h :: Type Void -> Int
h Void = g ()

h :: Type Int -> Int
h Int = 0

g () = f ()

_ = Type (f ()) :: Type Int
_ = Type (g ()) :: Type Int

[girving]

This is okay to do for now since I believe its safe for all "monotonic" functions, whose execution paths and intermediate types dependent monotonically on the types of other functions. This stuff will be correct once I finish the generic fixpoint code and switch inference to use it.