Add let generalization

[TimWhiting]

Currently Koka instantiates at lets, this causes issues with using a function at multiple different effects.

Making that small change would solve the following issues. #401 #402

Daan has been concerned whether this will cause other issues, due to some issues mentioned in this paper: Let should not be generalized

We should investigate whether Koka has the problems listed in the paper, and if not, let generalize, which would make the effect system a lot nicer.