Implement `PartialEq` and `Eq` for `Deque`

[hicklin]

Currently, Deque does not implement the PartialEq and Eq traits. This hinders structures that contain a Deque from being able to derive PartialEq and Eq. Equally, it requires library consumers to implement their own comparison algorithm. This algorithm can be implemented once in the library, improving the useability of the Deque.

The same algorithm used for the standard VecDeque could be implemented here.