A very modest typeclass caching

[mtzguido]

This PR makes the resolve_implicits loop remember the solutions of typeclass arguments and reuse when the type of the implicit and the environment exactly match. It's pretty conservative, but easy to argue for its soundness and simple to implement, without any type of global persistent state.

It doesn't do a big difference in most cases, though, in a snippet like:

  f (show x);
  g (show x)

we cannot reuse the solution since the environment in the second line is different, it is extended with an extra unit binding.