(grow! deq start-n end-n (? elem))
  deq: a deque
  start-n: an int
  end-n: an int
  elem: any type (optional)
  returns #n

Increases a deque's size.

start-n copies of elem are pushed to the start of the deque, and end-n copies of elem are push to the end of the deque.

When elem is absent, it defaults to #n for an array, or \0 for a string.