GHC Exactprint merge process

[Alan & Kim Zimmerman]

I have been running a branch in !2418[1] for just over a year to migrate
the ghc-exactprint functionality directly into the GHC AST[2], and I am now
satisfied that it is able to provide all the same functionality as the
original.

This is one of the features intended for the impending 9.2.1 release, and
it needs to be reviewed to be able to land.  But the change is huge, as it
mechanically affects most files that interact with the GHC AST.

So I have split out a precursor !5158 [3] with just the new types that are
used to represent the annotations, so it can be a focal point for
discussion.

It is ready for review, please comment if you have time and interest.

Regards
  Alan

[1] https://gitlab.haskell.org/ghc/ghc/-/merge_requests/2418
[2] https://gitlab.haskell.org/ghc/ghc/-/issues/17638
[3] https://gitlab.haskell.org/ghc/ghc/-/merge_requests/5158
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/ghc-devs/attachments/20210306/fb89262d/attachment.html>