Fixing type synonyms to Uniq(D)FM newtypes

[Andreas Klebinger]

There was a discussion about making UniqFM typed for the keys here a
while ago.
(https://mail.haskell.org/pipermail/ghc-devs/2020-January/018451.html
and following)

I wrote up an MR for one possible approach here:
https://gitlab.haskell.org/ghc/ghc/-/merge_requests/3577

It implements solution 2 from that discussion.

Just while getting the patch to typecheck I've already seen a number of
cases where this increased
readability of the code quite a bit so I think it's a good improvement.

If there are strong objections to this solution let me know. In that
case I'm happy to abandon the patch.
If not I will clean it up and get it ready for merging.