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/all-packages.nix

@@ -1,4 +1,6 @@
 self: pkgs: with pkgs; {
 
   cryptominisat = callPackage ./cryptominisat.nix { };
+
+  firedrake = callPackage ./firedrake { };
 }