The implementation is similar to the one from the double-conversion
library used in the Chrome V8 engine.

                            old ns/op   new ns/op  speedup
BenchmarkAppendFloatDecimal      591         480      1.2x
BenchmarkAppendFloat            2956         486      6.1x
BenchmarkAppendFloatExp        10622         503     21.1x
BenchmarkAppendFloatNegExp     40343         483     83.5x
BenchmarkAppendFloatBig         2798         664      4.2x

See F. Loitsch, ``Printing Floating-Point Numbers Quickly and
Accurately with Integers'', Proceedings of the ACM, 2010.

R=rsc
CC=golang-dev, remy
https://golang.org/cl/5502079