Refactor Nix flake

- add a Nix formatter
- add a default package & devShell.
- restructure the overlays
- move the inputs to the end
- don’t repeat the GHC version list
- move lib into its own file (and add more functions)
- add nixConfig
- provide a “Haskell overlay”
- disable GHC 9.4 and HEAD builds, as they fail
- enable all packages now that they work with newer GHC (except for concat-hardware)
- enable `checkPhase` for all Cabal packages
- replace string passing with a package set
- extract "ghc902" to `defaultCompiler`
- add a Garnix config to enable darwin builds
- expose executables from Cabal packages

Author: sellout