Adding (appending) new items to our queue

Remember two of our objectives:

• To quickly add items - we don't wish appending to a very long queue to be slow.
  We achieve this by remembering where the tail of the queue is, and quickly adding to it without searching.

• To be able to queue data that we don't "understand".
  We achieve this by treating all data as "a block of bytes", allocating memory for it, copying it (as we're told its length), all without ever interpreting its contents.