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!