Fix floating of equalities
This rather subtle patch fixes Trac #14584. The problem was that we'd allowed a coercion, bound in a nested scope, to escape into an outer scope. The main changes are * TcSimplify.floatEqualities takes more care when floating equalities to make sure we don't float one out that mentions a locally-bound coercion. See Note [What prevents a constraint from floating] * TcSimplify.emitResidualConstraints (which emits the residual constraints in simplifyInfer) now avoids burying the constraints for escaping CoVars inside the implication constraint. * Since I had do to this stuff with CoVars, I moved the fancy footwork about not quantifying over CoVars from TcMType.quantifyTyVars to its caller TcSimplify.decideQuantifiedTyVars. I think its other callers don't need to worry about all this CoVar stuff. This turned out to be surprisigly tricky, and took me a solid day to get right. I think the result is reasonably neat, though, and well documented with Notes.