Rework HTTP response logic to avoid allocation

[dead-claudia]

This builds on #48. My server is structured to use response templates, so it's very easy to avoid allocation.

I'd like to see a variant of request.respond(...) that doesn't require allocation (especially for headers), so I don't need to create an entire short-lived Vec just for headers. I was looking through the source code, and this seems very doable. The response logic just uses the Response object to respond directly without queueing anything. And the request.respond(response) method itself neither does any allocation nor does any queueing either.