strconv: make Eisel-Lemire handle long mantissas

strconv: make Eisel-Lemire handle long mantissas

In many cases, it is not necessary to parse long
decimal mantissas entirely to produce the correctly
rounded floating-point number. It is enough to parse
the short, rounded lower and upper bounds and in most cases
they round to the same floating point number because uint64
can hold 19 digits.

Previously this case was handled by the extFloat code path
(Grisu3 algorithm).

name                      old time/op  new time/op  delta
Atof64Big-4               1.07µs ± 2%  0.11µs ± 2%  -89.61%  (p=0.000 n=10+9)
Atof64RandomLongFloats-4  8.03µs ± 2%  0.14µs ± 7%  -98.24%  (p=0.000 n=10+10)
Atof32RandomLong-4         760ns ± 1%   156ns ± 0%  -79.46%  (p=0.000 n=10+8)

Benchmarks versus extFloat:

name                      old time/op  new time/op  delta
Atof64Big-4                121ns ± 3%   111ns ± 2%   -7.93%  (p=0.000 n=10+9)
Atof64RandomLongFloats-4   144ns ± 1%   142ns ± 7%     ~     (p=0.167 n=10+10)
Atof32RandomLong-4         129ns ± 1%   156ns ± 0%  +21.12%  (p=0.000 n=10+8)

Change-Id: Id734b8c11e74b49a444fda67ee72870ae9422e60
Reviewed-on: https://go-review.googlesource.com/c/go/+/264677
Trust: Emmanuel Odeke <emmanuel@orijtech.com>
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Emmanuel Odeke <emmanuel@orijtech.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Nigel Tao <nigeltao@golang.org>