We've noticed that `checkFamInstConsistency` takes 6% of
overall build time on our codebase.
I've poked around for a bit and most of type family
instances are `Rep` from `Generics`. I think those are
unavoidable, so I don't think we can have less of them.

I also looked at the code and noticed a simple algorithmic
improvement can be made. The algorithm is pretty simple:
we take all the family instances from one module (`M1`)
and test it against another module (`M2`).
The cost of that is dominated by the size of `M1`, because
for each instance in `M1` we look it up in the type family
env from `M2`, and lookup is cheap.
If `M1` is bigger than `M2`, that's suboptimal, so after
my change we always iterate through the smaller set.

This drives down the cost of `checkFamInstConsistency`
to 2%.

Test Plan: harbormaster

Reviewers: simonmar, simonpj, goldfire, rwbarton, bgamari, ezyang, austin

Reviewed By: rwbarton, ezyang

Subscribers: ezyang, thomie

Differential Revision: https://phabricator.haskell.org/D2833