Clean up Coercible handling, and interaction of data families with newtypes
This patch fixes Trac #9580, in which the Coercible machinery succeeded even though the relevant data constructor was not in scope. As usual I got dragged into a raft of refactoring changes, all for the better. * Delete TcEvidence.coercionToTcCoercion (now unused) * Move instNewTyConTF_maybe, instNewTyCon_maybe to FamInst, and rename them to tcInstNewTyConTF_maybe, tcInstNewTyCon (They both return TcCoercions.) * tcInstNewTyConTF_maybe also gets more convenient type, which improves TcInteract.getCoercibleInst * Define FamInst.tcLookupDataFamInst, and use it in TcDeriv, (as well as in tcInstNewTyConTF_maybe) * Improve error report for Coercible errors, when data familes are involved Another use of tcLookupDataFamInst * In TcExpr.tcTagToEnum, use tcLookupDataFamInst to replace local hacky code * Fix Coercion.instNewTyCon_maybe and Type.newTyConInstRhs to deal with eta-reduced newtypes, using (new) Type.unwrapNewTyConEtad_maybe and (new) Type.applyTysX Some small refactoring of TcSMonad.matchFam.