Add `pop()`, `appendleft()`, and `extend()` methods, support iteration
and indexing, and initializing from an existing sequence.
Iteration and indexing (subscription) have independent configuration flags
to enable them. They are enabled by default at the same level that
collections.deque is enabled (the extra features level).
Also add tests for checking new behavior.
Signed-off-by: Damien George <damien@micropython.org>
