Consider optimizations to reduce the number of host calls

[lukewagner]

While host calls are pretty fast these days, it's still useful to consider optimizations to reduce their number per request for really low-latency scenarios. This has come up in two areas so far:

• @PiotrSikora mentioned in this comment that many requests have empty bodies, so perhaps consume could short-circuit the hop through incoming-stream, maybe with a variant like this.
• @brendandburns mentioned in this comment the sequence of calls to create headers, then create a response, then make the request. Perhaps we could have some more one-shot options that do multiple of these at once.

Both seem like potentially good ideas to me but I'd be happy to hear more thoughts.

[brendandburns]

fwiw, part of the motivation to reduce host calls is also to improve the security of the surface area. I believe it is easier to ensure the security of a single req(...) style call vs all of the memory manipulation of the various calls between guest and host.

It's also a cleaner abstraction for the implementor, so fewer bugs in the implementation.