Rewrite Deque

[saeta]

The previous implementation did not appropriately conform to the
standard Collection protocols, and additionally had extra reference
counting operations. This new Deque implementation is designed to be
more flexible and easy to use without sacrificing performance.

[dabrahams]

@saeta You left a review request yet it's still a draft. Should I review or hold off?

[saeta]

@dabrahams Thanks for asking! I have a number of things I want to add (e.g. RandomAccessCollection conformance, MutableCollection conformance, etc), but the basics of the new Deque implementation should be there, and I'd definitely appreciate a review of those. (i.e. A "30%" review.)

[saeta]

One question for you I have is: I'm not sure how to satisfy both the requirements for Index on BidirectionalCollection (in particular, comparison between Indexes) as well as the design goal of having stable Indexes. Happy to chat further if that's helpful.