Adding a "close" method to `{Future,Stream}[Producer,Consumer}` traits

[alexcrichton]

@dicej in the fuzzer I'm writing I'm finding that it's difficult to know when a producer/consumer is dropped by the guest in the host. Currently this is only detect-able via Drop for MyThing which works well enough, but I was wondering if it would be viable to add a fn close(...) method on the traits? The trait method would take a StoreContextMut<D> which would enable updating store state on close which would be convenient. Is this easy enough to slot in to the implementation?

[dicej]

Yeah, that makes sense. I'll give it a shot today.