(shrink! deq start-n end-n)
  deq: a deque
  start-n: an int
  end-n: an int
  returns #n

Reduces a deque's size.

start-n elements are popped from the start of the deque, and end-n elements are popped from its end.

If the length of the deque is less than (+ start-n end-n), it's an error.