Cannot cancel a host-initiated read of a future

[alexcrichton]

Currently there's no API to, given a FutureReader, perform a read and then cancel that read to get back the original FutureReader.

[alexcrichton]

I'm realizing this is also applicable to streams. Another case that's not currently possible with streams is to read some items and then pass the stream back into wasm. Once a host starts reading a stream it can't stop at this time.

[alexcrichton]

To articulate this a bit more now that I've got more experience: here's a set of relate things you can't do with the current API:

• Start reading a {Future,Stream}Reader then cancel it to get back a {Future,Stream}Reader to pass back into wasm.
• Start reading a FutureReader and somehow return "closed" to the caller. The current API of FutureConsumer is such that "closed" is just not an option.