strconv: increase the Eisel-Lemire exp10 range

strconv: increase the Eisel-Lemire exp10 range

This grows the exp10 range for which the Eisel-Lemire algorithm applies
from [-307, +288] to [-348, +347], roughly equivalent to the existing
powersOfTen table in extfloat.go (which uses a different algorithm).

name                  old time/op  new time/op  delta
Atof64Decimal-4       48.4ns ± 1%  48.7ns ± 3%   ~     (p=0.698 n=5+5)
Atof64Float-4         57.9ns ± 1%  58.1ns ± 2%   ~     (p=0.873 n=5+5)
Atof64FloatExp-4      71.8ns ± 2%  72.2ns ± 2%   ~     (p=0.730 n=5+5)
Atof64Big-4            165ns ± 1%   164ns ± 1%   ~     (p=0.635 n=5+5)
Atof64RandomBits-4     165ns ± 1%   165ns ± 6%   ~     (p=0.143 n=5+5)
Atof64RandomFloats-4   147ns ± 2%   147ns ± 1%   ~     (p=0.857 n=5+5)

Change-Id: Idf7dc5297db6db2bd9e0bd4cb0e55e021916fa43
Reviewed-on: https://go-review.googlesource.com/c/go/+/264139
Reviewed-by: Robert Griesemer <gri@golang.org>
Trust: Robert Griesemer <gri@golang.org>
Trust: Nigel Tao <nigeltao@golang.org>