Lots of vehicle/riding related things, and some things that go boom

Adds 3 new events: entity_explode, vehicle_enter, and vehicle_collide. Vehicle_collide includes filters to prevent it from firing when a vehicle collides with the rider, as this isn't Garry's Mod, so we don't care about that, also to prevent it from firing for unsaddled pigs, as in that state they are no more a vehicle than any other mob. Vehicle_leave is not included because Bukkit doesn't fire it, so it would only cause confusion and silly bug reports.

Adds 3 new functions: get_entity_rider, get_entity_vehicle, and set_entity_stacking. The last of those is a more functional replacement for set_entity_rider, which has been moved out of sandbox and deprecated.

Fixes a spacing error in the docs for player_fish event.
Fixes set_objective_display not accepting enough args.
Adds the contents array to inventory_click event.