MicroHs support

Maybe you've already heard of [MicroHs](https://github.com/augustss/MicroHs). It is a small Haskell compiler written by @augustss using combinators. Since `primitive` is a dependency for many libraries, I'm interested in getting it to work with MicroHs. While `primitive` uses a lot of GHC primitives, MicroHs provides wrappers for some of them in [ghc-compat](https://github.com/augustss/ghc-compat/) (which is an implicit dependency for all MicroHs packages), and can be added as needed. MicroHs also has primitives for arrays etc.

Are you, the maintainers of `primitive`, interested in supporting MicroHs in this package? It would require some additional CPP and code, but a lot should be able to be covered by `ghc-compat`. The alternative is to fork `primitive` as `primitive-mhs` and modify MicroCabal (the build tool MicroHs uses) to always use the fork. This is already done for other packages (in particular `array` and `random`), so it wouldn't be difficult.

Either way, there is still some time until MicroHs can be supported, since it doesn't have `TypeFamilies` yet.