Replace implicit "response as stream" concept with an actual thing

A response should be a class with implicitish conversions from string, stream, buffer &c.. It would encapsulate status codes also. It's effectively going to end up a ~~monad with ```bind``` as "call handler on last response" and ```return``` as "coerce a string or whatever with status 200".~~ magma with addition as "do the handler function, munge the body, collapse the headers". Handlers will change from ```Response → Body → Body``` to ```Response → Response```.