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

@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` which would enable updating store state on close which would be convenient. Is this easy enough to slot in to the implementation?