Add queued()

[ghost]

Currently if a thread calls queueMain() with a function containing GCed memory then exits, it will be freed, causing an invalid memory access. This adds a function that allows threads to wait for all queued functions to be executed before exiting.

Usage:

proc start() =
  {.gcsafe.}:
    # ...
    while app.queued() > 0:
      discard