Add DerivingVia newtype for generic arbitrary

Add a newtype, GenericArbitrary, which can be used with -XDerivingVia to
derive Arbitrary instances for types with Generic, via patching
generic-arbitrary to expose the underlying typeclass it uses for
surfacing the type information.

haskell-overlay.nix

@@ -0,0 +1,7 @@
+{ nixpkgs ? import ./nixpkgs.nix {} }:
+let inherit (nixpkgs) pkgs;
+in self: super: rec {
+  generic-arbitrary = pkgs.haskell.lib.appendPatch
+    super.generic-arbitrary
+    [ ./generic-arbitrary-export-garbitrary.patch ];
+}