p3: rework consume-body

[rvolosatovs]

This PR addresses two gaps in current wasi:http:

• {request,response}::new return a future<result<_, error-code>>, but in case that consume-body is called on a request/response originating from this constructor, there is no way for the component to actually communicate a result on that future.
• consume-body is a method on request/response, but it can only be called once. That introduces complexity for the implementers and a potential hazard for the users, for example:

export wasi:http/handler#handle(req)
   res = wasi:http/handler#handle(req)
   res.consume_body()
   return res

Whether returning a response, body of which has been consumed, is valid or not is not obvious.

To address these issues:

• Take future<result<_, error-code>> as parameter in consume-body (see [v0.3]: Changes to consume-body #176 for more details)
• Make consume-body a static function moving the request/response it's called on.

A note on partial body read use case:
Some implementations may want to read some data from the body, but pass along the rest.
Such use cases are enabled with a pattern as follows:

export wasi:http/handler#handle(req)
   (tx, rx) = future()
   (body, trailers) = wasi:http/types.request#consume_body(req, rx)
   stream.read(body, 8) // read 8 bytes from body stream
   headers = wasi:http/types.fields()
   (res, result) = wasi:http/types.response#new(headers, Some(body), trailers, None)
   spawn(async {
      tx.send(result.await)
   })
   return res

Closes #176
Wasmtime implementation: bytecodealliance/wasmtime#11653