Deque impl.

[d coder]

> To be honest deque can be implemented far better if random access as in
> indexing, removal and such is dropped.

As other people too pointed out, Deque is indeed a random access
range, so indexing operator is a must. Insert/Remove too are good to
have. Array implementation in the std.container too implements remove
and insert.

Another small issue is that this Deque implementation is a Class.
std.container provides all the containers as structs. So for sake of
consistency, we should implement Deque too as a struct.

Regards
- Puneet