Refactor the hypothesis type in hls-tactics-plugin

[isovector]

The Hypothesis is all of the values in scope that the tactics engine knows about. It used to be a Map OccName HyInfo, mapping names to the information we have about them. But this is unhelpful for two reasons:

1. Almost nowhere in the code do we lookup by name
2. Class methods get instantiated for every concrete type, meaning we couldn't have two givens for different types in scope

Worse, it actively encouraged a bad style where we'd iterate through the keys of the hypothesis, and then need to look up the value in the map we were iterating. All of these callsites have now been simplified to just take the HyInfo directly, rather than an OccName that we'd subsequently need to lookup.

This PR refactors some of the underlying machinery. It's a bit simplistic now, but paves the way for a smarter implementation when I figure out what it should look like. In the meantime, it means we can now solve goals like

fmapBoth :: (Functor f, Functor g) => (a -> b) -> (f a, g a) -> (f b, g b)
fmapBoth = _

since the fmaps for f and g no longer clash.

[jneira]

Agree with the analysis and tests are green so lgtm