Deque.h - Extract away initialization logic of Deque.buff

[albertoCCz]

Initialization of Deque.buff is done in many places and might be convenient to move it to it's own function.

In principle, and without having examine it in detail, I think it might be enough to pass the start and end indexes of the buff to initialize as this chunk of memory should be always contiguous. Also, take care of empty_back and empty_head of Deque struct.

Possible prototype:
void __deque_initialize(Deque *deque, size_t start, size_t end)