The main motivator to do this now is some stubborn space leaks seem to be resolved.

Checklist:

• Get working with some minimal clean forks with PRs
• get feedback from dev team (do IDEs work?) EDIT: not much feedback, but did my best!
• upgrade CI (upgrade haskell-docker-packager?) , coordinate migration with devs
• shepherd remaining forks through upstream

Things we decided we can do post-merge:

• check out Nizar's benchmarks, quicjk before and after (*) EDIT: in progress
• Upgrade pro (NOTE: pro only depends on stable OSS hge releases, and stack supports 8.10 so we anticipate this being simple)

* There are some other performance-related things we'd like to experiment on, e.g. the new GC

EDIT: A few benchmarks

Performance is pretty comparable with what looks like some modest improvements. Cherry-picking a couple graphs. (Note: ghc 8.10 version here is compiled with -O1 since unfortunately I'm now getting occasional OOM compiling locally / O2 was taking a very long time for me; I guess we should at least open up a ticket):