Also: minor performance fix for large precision results.

benchmark                                   old ns/op    new ns/op    delta
strconv_test.BenchmarkFormatFloatDecimal         2734         2734   +0.00%
strconv_test.BenchmarkFormatFloat                3141         3139   -0.06%
strconv_test.BenchmarkFormatFloatExp             8970         8989   +0.21%
strconv_test.BenchmarkFormatFloatBig             3228         3208   -0.62%

Fixes #2535.

R=rsc
CC=golang-dev
https://golang.org/cl/5435089