fix subtle issue in tyvars_def

makes tyvars (Fun ty1 ty2) more uniform with the general case for a type
operator (since in the stateless world that's what it is).
prove tvars/tyvars_IMP and fill in constant case of proves_IMP

last remaining cheats are for the Abs case of INST_CORE_IMP and the
proves_IMP cases for the theorems about abs and rep that come out of a
type definition.