Make it compile with MicroHs

[konsumlamm]

Only depend on ghc-prim for GHC 9.0 (since it's only needed for importing Solo on base-4.15, which was shipped with GHC 9.0). MicroHs doesn't support older base versions and the code contains a lot of GHC-specific stuff, so guard it with __GLASGOW_HASKELL__.
I can add a MicroHs CI if you want, but I will also add base-orphans to MicroHs' CI, so that the latest (Stackage) release of base-orphans is always tested against the latest MicroHs build.

[RyanGlScott]

I fixed the bitrotten CI in 3d13612. Can you rebase your changes on top of that commit?

[RyanGlScott]

If I understand correctly, the approach here is to simply not define any instances at all when using non-GHC compilers. Is that correct?

What should be the story for supporting MicroHs versions of base going forward? Will MicroHs match future changes to GHC's version of base, or will MicroHs's version of base evolve independently? If it's the former, I wonder if the approach of not defining any orphan instances for MicroHs will work in the future.

[konsumlamm]

If I understand correctly, the approach here is to simply not define any instances at all when using non-GHC compilers. Is that correct?

Yes.

What should be the story for supporting MicroHs versions of base going forward? Will MicroHs match future changes to GHC's version of base, or will MicroHs's version of base evolve independently? If it's the former, I wonder if the approach of not defining any orphan instances for MicroHs will work in the future.

MicroHs will match future changes of GHC's base for the most part. It's still far from being stable, each release has a lot of new features and bug fixes, so at least currently, it doesn't make much sense to support older versions of MicroHs. Maybe @augustss can say more?

[augustss]

I want MicroHs to match base from GHC as best it can. It's not there yet.

[augustss]

And it's pointless to support older MicroHs version. There are not enough users.

[RyanGlScott]

Thanks! In that case, I'll go ahead and merge this as-is.