auto merge of #7562 : blake2-ppc/rust/deque, r=cmr

Fix an assertion in grow when using add_front.

Speeds up grow (and the functions that use it) by a lot. We retain the vector instead of creating it anew for each grow. 

The struct field hi is removed since it is redundant, and all iterators are updated to use a representation closer to the Deque itself, it should make it work even if deque sizes are not powers of two.

Deque::with_capacity is also implemented, but .reserve() is not yet done.

Author: bors