Yet another major refactoring of the constraint solver
This is the result of Simon and Dimitrios doing a code walk through. There is no change in behaviour, but the structure is much better. Main changes: * Given constraints contain an EvTerm not an EvVar * Correspondingly, TcEvidence is a recursive types that uses EvTerms rather than EvVars * Rename CtFlavor to CtEvidence * Every CtEvidence has a ctev_pred field. And use record fields consistently for CtEvidence * The solved-constraint fields of InertSet (namely inert_solved and inert_solved_funeqs) contain CtEvidence, not Ct There is a long cascade of follow-on changes.