Solved goal caching and zonking optimisations.

1) Stopped rewriting and caching solveds in the inerts because
profiling showed that a lot of time was spent on rewriting
already solved goals.

2) Optimisations in zonkEvBinds for common-case
evidence bindings generated from the constraint solver.

3) Now solved goals cache their evidence terms, so that we can more
aggressively optimize Refl coercions during constraint solving.
This patch also includes a rewrite of rewriteInertEqsFromInertEq
which greatly improves its efficiency.