Add GHC 9 support

[AriFordsham]

I've modified ghc-source-gen so it builds and passes tests on GHC 9.0.1. I hope you find this useful for #77.

It's not production-ready; I just 'brute-forced' it - I definitely broke GHC 8.10, and I just stubbed out any new features etc. with whatever seemed to make sense to allow building.

I had to comment out two tests to get it to build; all other tests are passing.

Main issues:

• HsTyVarBndr now has a Specificity flag
• linear arrows in `infixCon``
• Various pass-dependent things have been lifted to type-level and now have less fields. Especially troublesome are the ConPat constructor(s)

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[AriFordsham]

@googlebot I signed it!

[AriFordsham]

Please note I added a GHC 9 CI job and deleted the others.

[blackgnezdo]

Please note I added a GHC 9 CI job and deleted the others.

Is supporting at least ghc 8.10 too much trouble? I'm worried about single-target releases.

[blackgnezdo]

Please note I added a GHC 9 CI job and deleted the others.

Is supporting at least ghc 8.10 too much trouble? I'm worried about single-target releases.

I think I understand now, this PR shows what it takes to get ghc 9 compatibility. Somebody needs to finish the change so that multiple versions can coexist.

This makes sense, thanks for the PR!

[AriFordsham]

Please note I added a GHC 9 CI job and deleted the others.

Is supporting at least ghc 8.10 too much trouble? I'm worried about single-target releases.

I think I understand now, this PR shows what it takes to get ghc 9 compatibility. Somebody needs to finish the change so that multiple versions can coexist.

This makes sense, thanks for the PR!

I don't even know if new functionality is correct. I just know it builds.

[AriFordsham]

I think GHC 9 support #77 can be considered working experimentally at this stage: There's no new GHC 9 features, and I can't garantee I haven't broken anything making the neccesary GHC 9 changes. GHC 8 should be working unchanged.

[blackgnezdo]

Could you rebase on top of #85 and make the tests pass?

[AriFordsham]

I'll look at it sometime. I also remember that I left two test cases commented out - I should try to fix those, or at least document if I've broken the API.

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[AriFordsham]

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@blackgnezdo

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[blackgnezdo]

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Could you rebase on top of HEAD and force-push this to clear the complaint?

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[blackgnezdo]

@judah it would be great if you could look at the code as @AriFordsham has some semantic questions that will take me a while to get up to speed with.

[AriFordsham]

I think #86 is unequivocally better than this.