Build with MicroHs 0.15.x.x

[sjshuck]

Currently, mcabal install with the v0.14.15.0 toolchain works as is in the GitHub CI YAML, but v0.14.33.0 and v0.15.3.0 do not. They complain about a missing ghc-compat dependency.

Tangentially, are there any changes to transformers that would sidestep the need for ghc-compat? I don't know anything about that library.

att'n @augustss

[augustss]

I'll take a look and update to a newer toolchain.