io.WriteString: if the object has a WriteString method, use it
io.WriteString: if the object has a WriteString method, use it

This avoids allocation when writing to bytes.Buffers and bufio.Writers, for
example.

R=golang-dev, rsc, r, consalus, r
CC=golang-dev
http://codereview.appspot.com/4625068

Committer: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

tav/go,tav/go,tav/go,tav/go,tav/go,tav/go,tav/go