Add a `StaticVecDeque` type

Hi,

I see you also provide derived types such as `StaticHeap` and `StaticString`. What would you think about adding a `StaticVecDeque` type mimicking the standard [`VecDeque`](https://doc.rust-lang.org/std/collections/struct.VecDeque.html)? It would be super useful although it cannot be directly implemented from `StaticVec` since some elements on the front of the vec might be uninitialized...