General tidy up of dependencies and warnings

[tmcgilchrist]

Thank you for picking up maintenance of this package.

I've done some hopefully useful (and non-controversial) fixes to cabal files, source files and CI setup.
I don't think this introduces any regressions but I have seen intermittent failures on the testsuite that are fixable by re-running the tests. The test suite needs work to make it more reliable, which I've not done here.

• a1071e6 removes unnecessary package depdendencies.
• b601ed1 changes GH CI so it will only build once per PR. Previously it was running two identical builds for PRs.
• e3196db removes unnecessary imports, exports and variable bindings.

[LaurentRDC]

This is great! Thank you so much

[LaurentRDC]

I'm preparing releases for all packages with unreleased changes
Edit: changes published

[tmcgilchrist]

Thank you @LaurentRDC