v3

This PR includes a major change in the backing queue of the recursive concurrent iterator.

Prior queue is revised and preserved behind the "experimental" feature. The new backend is replaced with crossbeam-deque with "std" and with crossbeam-queue in no-std builds.

Benchmarks extended.

Extend signature is relaxed from requiring an exact size iterator to just an iterator. Internal extnsion still makes use of the known length when size_hint provides an exact length, falls back to the flexible extension otherwise.

Overall, these changes stabilize the recursive iterator, which was simpler; however, might lead to poor performance when consumed by many threads.