Bump nixpkgs and simplify/clean up nix setup

[monacoremo]

This is on top of #1528, will rebase. Update: rebased.

Now that we have a pretty complete Nix setup and gained some experience, I noticed some opportunities to simplify the Nix setup. Fixes include:

• Explicitly set the default Postgresql version we want to use in an overlay (this will also be the version that libpq is taken from)
• Explicitly specify the GHC version being used when accessing haskellPackages (this and the one above makes the setup more independent from changes in Nixpkgs)
• Merge the scripts related to development into one Nix file, devtools.nix - no real need to have them in separate files
• Merge the haskellPackages overlay for static and dynamic builds (should more maintainable, as changes don't need to be synced between the two anymore)
• Only have the major versions of the different Postgresql versions in the testing script names (i.e. 9.4 instead of 9.4.27, easier to use locally and potentially in CI)
• Add a recommendation for cachix use postgrest in the README
• Pin protolude using callHackageDirect, instead of creating a local file with cabal2nix

I also bumped Nixpkgs to the current nixpkgs-unstable version, which caused no major changes.* I think it's good if we track unstable closely for now, at least until all our patches and already merged PRs have landed in a stable version.

*edit: It did yield new hlint hints and nixpkgs-fmt formatting changes, both added in separate commits.

Those a quite a few changes, but as everything is related to the Nix setup I hope it's still good to review!

[steve-chavez]

Looks good!

I gave it a look and also tried the new postgrest-test-spec commands, I like the shorter version numbers.

Combining the static and dynamic build into a single config is great. Btw, maybe we can later leverage this to build a static bin for arm64. There was some work for this on #1117 (comment), but there were some issues.

Merging now!