Synchronize base, template-haskell changelogs with edits from ghc-8.8 branch
The changelogs for base and template-haskell in the master branch are slightly out of date with respect to their counterparts in the ghc-8.8 branch, as @bgamari applied some local edits to them after splitting off the ghc-8.8 that never got synced back to master. This PR cherry-picks the relevant three commits back to master:
I have also authored a fourth commit which fixes some italics formatting in base's changelog. This could be backported to ghc-8.8 if desired, but it's not critical.