strconv: put decimal on stack

strconv: put decimal on stack

This makes decimal a good test
case for the escape analysis.

With escape analysis:

benchmark                 old ns/op    new ns/op    delta
BenchmarkAtof64Decimal         1954          243  -87.56%
BenchmarkAtof64Float           2008          293  -85.41%
BenchmarkAtof64FloatExp       10106         8814  -12.78%
BenchmarkAtof64Big             5113         3486  -31.82%

R=golang-dev, gri
CC=golang-dev
https://golang.org/cl/4861042