Buffer intermediate output via a bytes.Buffer and thus avoid
calling through the entire Writer stack for every item printed.
There is more opportunity for improvements along the same lines.

Before (best of 3 runs):
- printer.BenchmarkPrint	      50	  47959760 ns/op
- time gofmt -l $GOROOT/src	      real	  0m11.517s

After (best of 3 runs):
- printer.BenchmarkPrint	      50	  32056640 ns/op (= -33%)
- time gofmt -l $GOROOT/src	      real	  0m9.070s       (= -21%)

R=r
CC=golang-dev
https://golang.org/cl/5432054