feat(p3): only allow body.stream calls once

[rvolosatovs]

Only allow body.stream to be called once.

This simplifies the host implementation and the amount of bookkeeping that would need to be performed by the runtime. I cannot really come up with a use case that would be impossible for guests to implement using the "at-most-once" call semantics.

We can always relax this requirement at a later point of time if it becomes an issue, but we can't quite go the other way round

This is aligned with e.g.:

• tcp-socket.receive https://github.com/WebAssembly/wasi-sockets/blob/41d707935af71418ea5477d5fc9c7281353fa7f5/wit-0.3.0-draft/types.wit#L326-L346
• tcp-socket.listen https://github.com/WebAssembly/wasi-sockets/blob/41d707935af71418ea5477d5fc9c7281353fa7f5/wit-0.3.0-draft/types.wit#L226-L289

[lukewagner]

Ah, the intended design (which could perhaps be documented more clearly) was that if you call %stream, then close it, then call %stream again, you'll resume where you left off (not get a copy of the contents since the beginning), up until you call finish. The idea was that this could be useful in chaining scenarios where component A consumes the first part (of a multi-part) request and then passes the request on to component B. But if it's a significant impl obstacle, happy to discuss here.