Integrate Persistent with Servant

Query my SQLite database from within my Servant handlers. Nothing I've written
is domain-specific to the business logic yet -- I'm just making sure everything
integrates.

shell.nix

@@ -8,6 +8,9 @@ in pkgs.mkShell {
       hpkgs.resource-pool
       hpkgs.sqlite-simple
       hpkgs.warp
+      hpkgs.persistent
+      hpkgs.persistent-sqlite
+      hpkgs.persistent-template
     ]))
   ];
 }