auto merge of #7944 : blake2-ppc/rust/dlist-move-nodes, r=bblum

Factor out internal methods to pop/push list nodes so that .merge() and .rotate_to_front(), .rotate_to_back() (new methods) can be implemented without allocating nodes.

With that, some cleanup changes to DList use of Option, and adding a missing Encodable implementation.

Author: bors