strconv: implement fast path for rounding already short numbers.
strconv: implement fast path for rounding already short numbers.

benchmark                   old ns/op   new ns/op   delta
BenchmarkFormatFloatDecimal      3765        1386    -63%

R=rsc
CC=golang-dev, remy
http://codereview.appspot.com/5494060

Committer: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

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