Move-to-front heuristic

After a successful search in a linked list, detach the found linked list node and insert that same linked list node at the front. An immediate repetition of that search then succeeds after one key comparison. This helps workloads with temporal locality, though it need not reduce the cost of the search that triggers the move.

A dummy header node lets a search return the found node's predecessor. If that predecessor is already the header, do nothing; otherwise bypass the found node, point it at the old first real node, and point the header at it. This preserves each key/value binding and avoids allocation. It preserves a dictionary abstraction whose ordering is unobservable, but generally changes the abstraction of an ordered list.