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; and
- expose executables from Cabal packages.

Author: sellout