Fix bogus worker for newtypes
The "worker" for a newtype is actually a function
with a small (compulsory) unfolding, namely a cast.
But the construction of this function was plain wrong
for newtype /instances/; it cast the arguemnt to the
family type rather than the representation type.
This never actually bit us because, in the case of a
family instance, we immediately cast the result to
the family type. So we get
\x. (x |> co1) |> co2
where the compositio of co1 and co2 is ill-kinded.
However the optimiser (even the simple optimiser)
just collapsed those casts, ignoring the mis-match
in the middle, so we never saw the problem.
This patch fixes the ill-typed casts, properly. I can't
see a way to trigger an actual failure prior to this
patch, but it's still wrong wrong wrong to have ill-typed
casts, so better to get rid of them.