Fixing type synonyms to Uniq(D)FM newtypes

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.