Major refactoring of CoAxioms
This patch should have no user-visible effect. It implements a significant internal refactoring of the way that FC axioms are handled. The ultimate goal is to put us in a position to implement "pattern-matching axioms". But the changes here are only does refactoring; there is no change in functionality. Specifically: * We now treat data/type family instance declarations very, very similarly to types class instance declarations: - Renamed InstEnv.Instance as InstEnv.ClsInst, for symmetry with FamInstEnv.FamInst. This change does affect the GHC API, but for the better I think. - Previously, each family type/data instance declaration gave rise to a *TyCon*; typechecking a type/data instance decl produced that TyCon. Now, each type/data instance gives rise to a *FamInst*, by direct analogy with each class instance declaration giving rise to a ClsInst. - Just as each ClsInst contains its evidence, a DFunId, so each FamInst contains its evidence, a CoAxiom. See Note [FamInsts and CoAxioms] in FamInstEnv. The CoAxiom is a System-FC thing, and can relate any two types, whereas the FamInst relates directly to the Haskell source language construct, and always has a function (F tys) on the LHS. - Just as a DFunId has its own declaration in an interface file, so now do CoAxioms (see IfaceSyn.IfaceAxiom). These changes give rise to almost all the refactoring. * We used to have a hack whereby a type family instance produced a dummy type synonym, thus type instance F Int = Bool -> Bool translated to axiom FInt :: F Int ~ R:FInt type R:FInt = Bool -> Bool This was always a hack, and now it's gone. Instead the type instance declaration produces a FamInst, whose axiom has kind axiom FInt :: F Int ~ Bool -> Bool just as you'd expect. * Newtypes are done just as before; they generate a CoAxiom. These CoAxioms are "implicit" (do not generate an IfaceAxiom declaration), unlike the ones coming from family instance declarations. See Note [Implicit axioms] in TyCon On the whole the code gets significantly nicer. There were consequential tidy-ups in the vectoriser, but I think I got them right.