Implement Storage with VecDeque or similar

[flo-l]

Storage is currently implemented using a Vec as a backing storage. This becomes inefficient for huge window sizes, as the whole array needs to be shifted on every call to next() sometimes.

As noted in #1 it's very hard to implement Storage generically over any container.

• One would lose the ability of Window to deref to a (mut) slice.
• Window probably has to implement Iterator or Index/IndexMut.

I'd be happy to merge a PR that implements Storage generically over containers if good solutions to the above problems are found and if no functionality gets lost.

Anyway I'd also merge a PR implementing another adaptor, which uses a VecDeque or anything efficient for huge window sizes.

[flo-l]

Well, v3.0 and above break the old API to implement this but better. Instead of VecDeque we have Vec with custom Iterators.