Checkpoint
A checkpoint for one player, owned by the resource that created it (removed when that resource stops or the player leaves). Raises checkpoint_enter and checkpoint_leave. Reads on a removed checkpoint return nil, err.
Checkpoint:id
Returns id integer
Checkpoint:is_inside
Whether its player is currently inside.
Returns inside boolean|nil, err string|nil
Checkpoint:player
Returns player Player|nil, err string|nil
