gob: avoid one copy for every message written.

gob: avoid one copy for every message written.
Plus the need for a second in-memory buffer.
Plays a bit fast and loose with the contents of a byte buffer,
but saves a potentially huge allocation. The gotest
run is about 10% faster overall after this change.

R=golang-dev, r, gri
CC=golang-dev
https://golang.org/cl/5236043