Addition of continuation-passing-style arguments to all queueing methods.

With this change, all methods that queue an action now take an optional last argument that is a function that is called after the queued action is completed. (This includes all turtle motion, pen, speed, fd, bk, rt, lt, moveto, etc.)

This allows a user of iced coffeescript to say "await fd 100, defer()" and wait for the action to be finished before going on to the next step.