Changes to avoid GHC warnings on building

[mpilgrem]

These changes avoid almost all GHC warnings on building. The one warning I could not work out how to avoid was:

<no location info>: warning: [-Wunused-packages]
    The following packages were specified via -package or -package-id flags,
    but were not needed for compilation:
      - base-4.17.2.0 (exposed by flag -package-id base-4.17.2.0)

[tomjaguarpaw]

Thanks! I'm not a big fan of making effort to avoid warnings, to be honest, but I'll take most of these. A couple I'd like to undo and rethink.

[mpilgrem]

@tomjaguarpaw, hopefully this now reflects what you were looking for.

[tomjaguarpaw]

Thanks! Set to automerge. Feel free to make to submit the other changes as a follow-up PR. I'll give them some thought.