More intelligent binding suggestions

[cb372]

In the Relevant bindings include... section, currently we list all the bindings we can find. It would be nice to use the type of the hole to prioritise these bindings in some way, and maybe filter out any that are clearly irrelevant.

There's a wide spectrum of how clever we want to be with this. We need to be careful not to accidentally write our own typechecker or go too far down the road of Idris-style proof search or Curry-Howard correspondence. I'd like to see how far we could get with just some simple heuristics first.