Module Queue: firstin firstout queues

This module implements queues (FIFOs), with inplace modification.
type 'a t

The type of queues containing elements of type 'a.
exception Empty

Raised when take is applied to an empty queue.
val create: unit > 'a t

Return a new queue, initially empty.
val add: 'a > 'a t > unit

add x q adds the element x at the end of the queue q.
val take: 'a t > 'a

take q removes and returns the first element in queue q,
or raises Empty if the queue is empty.
val peek: 'a t > 'a

peek q returns the first element in queue q, without removing
it from the queue, or raises Empty if the queue is empty.
val clear : 'a t > unit

Discard all elements from a queue.
val length: 'a t > int

Return the number of elements in a queue.
val iter: ('a > unit) > 'a t > unit

iter f q applies f in turn to all elements of q,
from the least recently entered to the most recently entered.
The queue itself is unchanged.