Allow full constraint solving under a for-all (Trac #5595)
The main idea is that when we unify forall a. t1 ~ forall a. t2 we get constraints from unifying t1~t2 that mention a. We are producing a coercion witnessing the equivalence of the for-alls, and inside *that* coercion we need bindings for the solved constraints arising from t1~t2. We didn't have way to do this before. The big change is that here's a new type TcEvidence.TcCoercion, which is much like Coercion.Coercion except that there's a slot for TcEvBinds in it. This has a wave of follow-on changes. Not deep but broad. * New module TcEvidence, which now contains the HsWrapper TcEvBinds, EvTerm etc types that used to be in HsBinds * The typechecker works exclusively in terms of TcCoercion. * The desugarer converts TcCoercion to Coercion * The main payload is in TcUnify.unifySigmaTy. This is the function that had a gross hack before, but is now beautiful. * LCoercion is gone! Hooray. Many many fiddly changes in conssequence. But it's nice.