Currently go fmt formats all files sequentially.
That's a shame. Parallelize it over files.

Reduces time of go fmt ./... in std lib
from ~6.1s to ~0.9s.

Reduces time of go fmt github.com/google/syzkaller/...
from ~5.2s to ~1.8s.

Change-Id: I3d27fc25326106b2a4781e13506a25c12d5bcdc5
Reviewed-on: https://go-review.googlesource.com/45491
Run-TryBot: Dmitry Vyukov <dvyukov@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>