No functional change; this pulls out all of the changes except the `polysemy`
refactoring from here:
https://github.com/ryantm/nixpkgs-update/pull/172
This will allow us to do development with the newer GHC and cleaned up
dependency stack, start using the `hspec` test suite, vet the `github` library
updates, etc., while we work on porting more of the effects to polysemy and
evaluating how it looks.
