strconv: faster float conversion
- added AppendFloatX benchmarks
- 2% to 13% better performance
- check for illegal bitSize
benchmark old ns/op new ns/op delta
strconv_test.BenchmarkFormatFloatDecimal 2993 2733 -8.69%
strconv_test.BenchmarkFormatFloat 3384 3141 -7.18%
strconv_test.BenchmarkFormatFloatExp 9192 9010 -1.98%
strconv_test.BenchmarkFormatFloatBig 3279 3207 -2.20%
strconv_test.BenchmarkAppendFloatDecimal 2837 2478 -12.65%
strconv_test.BenchmarkAppendFloat 3196 2928 -8.39%
strconv_test.BenchmarkAppendFloatExp 9028 8773 -2.82%
strconv_test.BenchmarkAppendFloatBig 3151 2782 -11.71%
R=rsc, bradfitz
CC=golang-dev
https://golang.org/cl/
5448122