New implementation for `ImpredicativeTypes`

[Alejandro Serrano Mena]

Hi all,
As I mentioned some time ago, we have been busy working on a new
implementation of `ImpredicativeTypes` for GHC. I am very thankful to
everybody who back then sent us examples of impredicativity which would be
nice to support, as far as we know this branch supports all of them! :)

If you want to try it, at
https://gitlab.haskell.org/trupill/ghc/commit/a3f95a0fe0f647702fd7225fa719a8062a4cc0a5/pipelines?ref=quick-look-build
you can find the result of the pipeline, which includes builds for several
platforms (click on the "Artifacts" button, the one which looks like a
cloud, to get them). The code is being developed at
https://gitlab.haskell.org/trupill/ghc.

Any code should run *unchanged* except for some eta-expansion required for
some specific usage patterns of higher-rank types. Please don't hesitate to
ask any questions or clarifications about it. A merge request for tracking
this can be found at https://gitlab.haskell.org/ghc/ghc/merge_requests/1659

Kind regards,
Alejandro
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