append!, prepend!, and other functions on deques

I'm working on a program which uses a queue of bytes, with the bytes going in at one end in groups of 1-8 kB and coming out the other in groups of 4 bytes, or vice versa. I figured a deque would be a good data structure for this. However, I can't push or pop more than one byte at a time, so I'll have to use loops. Could you add append! and prepend! functions to Deque, a way to get a subscript range of a deque as a vector, and a way to delete multiple elements at once from either end of a Deque?

Locate the Deque implementation and its existing push/pop and indexing tests; start by tracing those APIs and their edge cases. Done means Deque supports append!, prepend!, ranged access returning a vector, and removing multiple elements from either end, with tests covering the requested operations.