Multithreading

Created by: acook

Adds the following operations:

• co : runs another file in its own thread with its own $stack and returns two queues for communication
• work : runs a WV in its own thread with its own $stack, takes two queues for communication
• bkg : runs a WV in its own thread, takes the top item with it, put it onto its own new $stack
• wait : waits for all threads to complete before continuing
  • only useful in the main thread
  • you're usually better off using a Q if you need this functionality