firedrake: Moved to new overlay system and shared sub-scope

Firedrake contains a ton of inter-connected dependencies, many of
which are either firedrake specific or forked versions of upstream
packages.  Cleaner to create a firedrake sub-scope for all these.

pkgs/firedrake/fiat.nix

@@ -0,0 +1,25 @@
+{ lib, fetchFromGitHub, pythonPackages }:
+
+pythonPackages.buildPythonPackage rec {
+  version = "20847cdee0d323726571324f565ff3da6f19e700";
+  name = "firedrake-fiat-${version}";
+
+  src = fetchFromGitHub {
+    owner = "firedrakeproject";
+    repo = "fiat";
+    rev = "${version}";
+    sha256 = "1id0p06m7mwx8caz30g0cgci2n73mvqf6i6hnm1w08m605fxvwwf";
+  };
+
+  propagatedBuildInputs = [
+    pythonPackages.numpy
+    pythonPackages.sympy
+    pythonPackages.six
+  ];
+
+  meta = with lib; {
+    homepage = "https://github.com/firedrakeproject/fiat";
+    description = "Copy of upstream FIAT for use with Firedrake.";
+    license = licenses.lgpl3;
+  };
+}