Motivation for this change

loeOf isn't used anywhere in nixpkgs and it's broken. In particular it doesn't support element of type X:

$ nix-instantiate --eval -E 'with import <nixpkgs/lib>; (evalModules {
  modules = singleton { 
    options.derp = mkOption { 
      type = types.loeOf types.int;
      default = 23;
    };
  };
}).config.derp'
error: The option `derp' does not have a string value, in `<unknown-file>'.

The type could probably be fixed by replacing isString with elemType.check, but then, it's better to have less code, right? :)

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • Linux
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.