Also improve extfloat.Normalize to obtain a modest performance
gain in parsing, and add a shortcut path for exact integers.

benchmark                              old ns/op    new ns/op    delta
BenchmarkAtof64Decimal                        73           73   -0.54%
BenchmarkAtof64Float                          91           91   -0.54%
BenchmarkAtof64FloatExp                      198          180   -9.09%
BenchmarkAtof64Big                           307          308   +0.33%

BenchmarkAtof32Decimal                        72           72   +0.42%
BenchmarkAtof32Float                          83           83   -0.72%
BenchmarkAtof32FloatExp                      212          186  -12.26%
BenchmarkAtof32Random                        262          250   -4.58%

BenchmarkAppendFloatDecimal                  474          305  -35.65%
BenchmarkAppendFloat                         497          489   -1.61%
BenchmarkAppendFloatExp                      493          483   -2.03%
BenchmarkAppendFloatNegExp                   481          481   +0.00%
BenchmarkAppendFloatBig                      667          652   -2.25%

BenchmarkAppendFloat32Integer                338          307   -9.17%
BenchmarkAppendFloat32ExactFraction          364          439  +20.60%
BenchmarkAppendFloat32Point                 1299          490  -62.28%
BenchmarkAppendFloat32Exp                   2593          489  -81.14%
BenchmarkAppendFloat32NegExp                5116          481  -90.60%

R=rsc, r
CC=golang-dev, remy
https://golang.org/cl/6303087