This moves DumpRequest, DumpResponse, NewChunkedReader,
and NewChunkedWriter out of http, as part of the continued
http diet plan.

Also, adds DumpRequestOut (for dumping outbound requests),
since DumpRequest's ambiguity (the "wire representation" in
what direction?) was often a source of confusion and bug
reports.

R=rsc, adg
CC=golang-dev
https://golang.org/cl/5339041