mhs compatibility

[L0neGamer]

Proposed changlog entry below.

In this PR I add basic support for mhs's Solo, and fiddle with the Hashable instance generation so that it'll work for mhs.
I don't add mhs to the ci like QuickCheck does because their thing seems to be an odd script; I can look into that in future if that would be appreciated.

This is part of my ongoing work with nick8325/quickcheck#472.

Changelog entry:

• Add microHS support

[phadej]

I'm not interested in supporting MicroHS. I'm sorry.

[phadej]

I however will change conditionals in .cabal file to not explicitly refer to GHC either: #34 that should work for MicroHs too, if their base is the same (enough) as one GHC ships.

[L0neGamer]

I think one of the issues was the template haskell dependencies were also blocking, but I appreciate the refactor.

[phadej]

template haskell dependencies

That's something MicroHS developers have to solve. I won't add if !impl(ghc) buildable: False to TH using components. The build system should be able to allow me to declare that declaratively without hunting down which compilers support it or not. (FWIW, not all GHCs support splicing TH either, so it's not a new problem; it just was swiped under the rug since TH was invented).