Implement constructor for Deque from iterable

At present a Deque (and Stack/Queue) can only be created empty.

A constructor method should be added to enable the creation of a Deque from an iterable such as an array.
At present Deque([1,2,3]) errors.

A few issues to discuss

• What Deque Block Size to choose by default when given an array/iterable?
• Should a user be able to pass both an array and a blocksize? (at the moment Deque(x::Int) creates an empty Deque with block size of x). We could add a Deque(array,blocksize) method or similar to allow specification.

Start by inspecting the existing Deque constructor, especially the Deque(x::Int) form, and the related Stack and Queue constructors. Determine the default block size and whether an iterable may be combined with an explicit block size; done means iterable-based construction works for Deque and the related types with documented, tested behavior.