Shell Turner

Results 2 issues of Shell Turner

The automatically-derived Clone trait on MPMCFutSender (and I assume other Senders) requires that T be Clone - however this is unnecessary since no T is never actually cloned while cloning...

Fixes #1 Coco implements an epoch GC which calls destructors - this therefore shouldn't leak memory, and is properly lock-free for reads. See crossbeam-rs/crossbeam#134 for some discussion of Coco's epoch...