Documentation for Nix.Eval

[expipiplus1]

It would be nice to have some documentation and maybe an example of how to use Nix.Eval

[jwiegley]

This definitely needs documentation, as things are getting a bit complex. I'm moving toward a scheme where you can either implement full tree evaluation yourself, or just make an instance of MonadEval, which lets the core evaluator in Eval.hs resolve all of the static name binding for you.