strconv: implement Ryū-like algorithm for fixed precision ftoa

This patch implements a simplified version of Ulf Adams,
"Ryū: Fast Float-to-String Conversion" (doi:10.1145/3192366.3192369)
for formatting floating-point numbers with a fixed number of decimal
digits.

It uses the same principles but does not need to handle
the complex task of finding a shortest representation.
This allows to handle a few more cases than Grisu3, notably
formatting with up to 18 significant digits.

name                         old time/op  new time/op  delta
AppendFloat/32Fixed8Hard-4   72.0ns ± 2%  56.0ns ± 2%  -22.28%  (p=0.000 n=10+10)
AppendFloat/32Fixed9Hard-4   74.8ns ± 0%  64.2ns ± 2%  -14.16%  (p=0.000 n=8+10)
AppendFloat/64Fixed1-4       60.4ns ± 1%  54.2ns ± 1%  -10.31%  (p=0.000 n=10+9)
AppendFloat/64Fixed2-4       66.3ns ± 1%  53.3ns ± 1%  -19.54%  (p=0.000 n=10+9)
AppendFloat/64Fixed3-4       61.0ns ± 1%  55.0ns ± 2%   -9.80%  (p=0.000 n=9+10)
AppendFloat/64Fixed4-4       66.9ns ± 0%  52.0ns ± 2%  -22.20%  (p=0.000 n=8+10)
AppendFloat/64Fixed12-4      95.5ns ± 1%  76.2ns ± 3%  -20.19%  (p=0.000 n=10+9)
AppendFloat/64Fixed16-4      1.62µs ± 0%  0.07µs ± 2%  -95.69%  (p=0.000 n=10+10)
AppendFloat/64Fixed12Hard-4  1.27µs ± 1%  0.07µs ± 1%  -94.83%  (p=0.000 n=9+9)
AppendFloat/64Fixed17Hard-4  3.68µs ± 1%  0.08µs ± 2%  -97.86%  (p=0.000 n=10+9)
AppendFloat/64Fixed18Hard-4  3.67µs ± 0%  3.72µs ± 1%   +1.44%  (p=0.000 n=9+10)

Updates #15672

Change-Id: I160963e141dd48287ad8cf57bcc3c686277788e8
Reviewed-on: https://go-review.googlesource.com/c/go/+/170079


Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Trust: Emmanuel Odeke <emmanuel@orijtech.com>
Trust: Nigel Tao <nigeltao@golang.org>
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Emmanuel Odeke <emmanuel@orijtech.com>
TryBot-Result: Go Bot <gobot@golang.org>