Fix build with mtl-2.3

[9999years]

mtl-2.3 no longer reexports Control.Monad

[mjrussell]

Thanks for the PR! Because this project uses a stack.yaml and the LTS is set to 20.17, the MTL package version is already pinned to 2.2.2 (at least if you are building with stack, there's no cabal version bounds). If we want to release a new version for support in a later LTS, we should bump the stack snapshot first and fix any errors that appear through that.

It looks like there isn't any active snapshot that I see with MTL 2.3...we might need to wait for that.

Alternatively we could use CPP here to deal with the breaking changes

[mjrussell]

Oh, just realized since these are re-export of same function it should be ok. Let me just run this through CI and might be able to merge it

[mjrussell]

All good, thanks again!