Add DequeView, similar to VecView and StringView

Ideally we would be able to merge the iterators of `&(mut) DequeView` and `&(mut) Deque`, but this can't be done without a breaking change.

Author: sosthene-nitrokey

CHANGELOG.md

@@ -17,6 +17,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 - Added `Extend` impls for `Deque`.
 - Added `Deque::make_contiguous`.
 - Added `VecView`, the `!Sized` version of `Vec`.
+- Added `DequeView`, the `!Sized` version of `Deque`.
 
 ### Changed