A radical overhaul of the coercion infrastucture
* Core Lint now does full checking of kinds and coercion terms which picks up kind errors in coercions that were previously simply not checked for * Coercion.lhs now provides optCoercion which optimises coercion terms. It implements all of Dimitrios's rules * The constructors for coercion terms now make no attempt to be "smart"; instead we rely solely on the coercion optimiser * CoercionTyCons in TyCon.lhs always had a "custom" kinding rule (the coKindFun field of CoercionTyCon) but its type was not clever enough to do both (a) *figure out the result kind*, assuming the whole thing is well-kinded in the first place (b) *check* the kinds of everything, failing gracefully if they aren't right. We need (b) for the new CoreLint stuff. The field now has type CoTyConKindChecker which does the job nicely.