strconv: speedup AppendFloat/FormatFloat.
The improvement is obtained by eliminating the zero
initialization of a large structure that is only
needed when the fast path fails.
Also add a missing roundtrip test for float32s.
benchmark old ns/op new ns/op delta
BenchmarkAppendFloatDecimal 301 180 -40.20%
BenchmarkAppendFloat 486 388 -20.16%
BenchmarkAppendFloatExp 492 383 -22.15%
BenchmarkAppendFloatNegExp 478 370 -22.59%
BenchmarkAppendFloatBig 650 541 -16.77%
BenchmarkAppendFloat32Integer 308 180 -41.56%
BenchmarkAppendFloat32ExactFraction 449 333 -25.84%
BenchmarkAppendFloat32Point 494 390 -21.05%
BenchmarkAppendFloat32Exp 488 387 -20.70%
BenchmarkAppendFloat32NegExp 488 378 -22.54%
R=r, rsc
CC=golang-dev, remy
https://golang.org/cl/
6346081