Support a basic API

Use Servant to create a REST API supporting the following routes:
- GET /number
- POST /other

The server interacts with a SQLite database.

shell.nix

@@ -3,6 +3,11 @@ let
 in pkgs.mkShell {
   buildInputs = with pkgs; [
     (haskellPackages.ghcWithPackages (hpkgs: with hpkgs; [
+      hpkgs.servant-server
+      hpkgs.aeson
+      hpkgs.resource-pool
+      hpkgs.sqlite-simple
+      hpkgs.warp
     ]))
   ];
 }