strconv: implement faster parsing of decimal numbers.
The algorithm is the same as in the double-conversion library
which also implements Florian Loitsch's fast printing algorithm.
It uses extended floats with a 64-bit mantissa, but cannot give
an answer for all cases.
old ns/op new ns/op speedup
BenchmarkAtof64Decimal 332 322 1.0x
BenchmarkAtof64Float 385 373 1.0x
BenchmarkAtof64FloatExp 9777 419 23.3x
BenchmarkAtof64Big 3934 691 5.7x
BenchmarkAtof64RandomBits 34060 899 37.9x
BenchmarkAtof64RandomFloats 1329 680 2.0x
See F. Loitsch, ``Printing Floating-Point Numbers Quickly and
Accurately with Integers'', Proceedings of the ACM, 2010.
R=ality, rsc
CC=golang-dev, remy
https://golang.org/cl/
5494068