Factor loading code out of CodeCity.startup
Factor loading code out of CodeCity.startup

Create CodeCity.loadCheckpoint to handle creating an Interpreter from a .city checkpoint, and CodeCity.loadStartup to handle creating an Interpreter instance pre-loaded with a directory of .js startup scripts.

NeilFraser/CodeCity,NeilFraser/CodeCity,google/CodeCity,google/CodeCity,google/CodeCity,NeilFraser/CodeCity,google/CodeCity,NeilFraser/CodeCity