Make code amenable to escape analysis
so that the decimal values do not escape.

benchmark                               old ns/op    new ns/op    delta
strconv_test.BenchmarkAtof64Decimal           229          233   +1.75%
strconv_test.BenchmarkAtof64Float             261          263   +0.77%
strconv_test.BenchmarkAtof64FloatExp         7760         7757   -0.04%
strconv_test.BenchmarkAtof64Big              3086         3053   -1.07%
strconv_test.BenchmarkFtoa64Decimal          6866         2629  -61.71%
strconv_test.BenchmarkFtoa64Float            7211         3064  -57.51%
strconv_test.BenchmarkFtoa64FloatExp        12587         8263  -34.35%
strconv_test.BenchmarkFtoa64Big              7058         2825  -59.97%
json.BenchmarkCodeEncoder               357355200    276528200  -22.62%
json.BenchmarkCodeMarshal               360735200    279646400  -22.48%
json.BenchmarkCodeDecoder               731528600    709460600   -3.02%
json.BenchmarkCodeUnmarshal             754774400    731051200   -3.14%
json.BenchmarkCodeUnmarshalReuse        713379000    704218000   -1.28%
json.BenchmarkSkipValue                  51594300     51682600   +0.17%

benchmark                                old MB/s     new MB/s  speedup
json.BenchmarkCodeEncoder                    5.43         7.02    1.29x
json.BenchmarkCodeMarshal                    5.38         6.94    1.29x
json.BenchmarkCodeDecoder                    2.65         2.74    1.03x
json.BenchmarkCodeUnmarshal                  2.57         2.65    1.03x
json.BenchmarkCodeUnmarshalReuse             2.72         2.76    1.01x
json.BenchmarkSkipValue                     38.61        38.55    1.00x

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5369111