A nice tidy-up for CvSubst and liftCoSubst
A "lifting substitition" takes a *type* to a *coercion*, using a substitution that takes a *type variable* to a *coercion*. We were using a CvSubst for this purpose, which was an awkward exception: in every other use of CvSubst, type variables map only to types. Turned out that Coercion.liftCoSubst is quite a small function, so I rewrote it with a special substitution type Coercion.LiftCoSubst, just for that purpose. In doing so I found that the function itself was bizarrely over-complicated ... a direct result of mis-using CvSubst. So this patch makes it all simpler, faster, and easier to understand. No bugs fixed though!