Vehicle JSON raw -> json conversion

Moves vehicles.json & vehicle_parts.json over to data/json directory
Altered game::game() to just do its immediate init stuff and then return the new game object. Requires that any calls to new game now be followed by a call to game::init_data. This change was made so that vehicles wouldn't freak out due to game not being fully initialized.
Gave vehicles and vehicle parts JsonObject loading functions. Vehicles have to be cached until after itypes is initialized, so have an extra function game::finalize_vehicles which gets called after init_itypes.

-- Now that PR #3449 has been merged, I updated to use the new layering functionality in the loading system. Should be ready to go now.