strconv: replace Ryu ftoa with Dragonbox

Dragonbox is a faster ftoa algorithm that provides the same guarantees
as Ryu: round-trip conversion, shortest length, and correct rounding.
The author of the reference implementation has agreed to distribute
this file under Go's BSD-style license.

The new implementation has been fuzz-tested against the current
Ryu implementation in addition to the existing test suite.
Benchmarks show at least ~15-20% performance improvement.

Dragonbox only supports shortest-precision conversion, so we continue to
use Ryu-printf for fixed precision.

The following shows the relevant output from benchstat. Full benchmark
results and plots are available at:
https://github.com/taichimaeda/dragonbox-bench/

goos: darwin
goarch: arm64
pkg: strconv
cpu: Apple M1
                                    │   old.txt    │               new.txt                │
                                    │    sec/op    │    sec/op     vs base                │
FormatFloat/Decimal-8                 32.71n ± 14%   31.89n ± 12%        ~ (p=0.165 n=10)
FormatFloat/Float-8                   45.54n ±  1%   42.48n ±  0%   -6.70% (p=0.000 n=10)
FormatFloat/Exp-8                     50.06n ±  0%   32.27n ±  1%  -35.54% (p=0.000 n=10)
FormatFloat/NegExp-8                  47.15n ±  1%   31.33n ±  0%  -33.56% (p=0.000 n=10)
FormatFloat/LongExp-8                 46.15n ±  1%   43.66n ±  0%   -5.38% (p=0.000 n=10)
FormatFloat/Big-8                     50.02n ±  0%   39.36n ±  0%  -21.31% (p=0.000 n=10)
FormatFloat/BinaryExp-8               27.89n ±  0%   27.88n ±  1%        ~ (p=0.798 n=10)
FormatFloat/32Integer-8               31.41n ±  0%   23.00n ±  3%  -26.79% (p=0.000 n=10)
FormatFloat/32ExactFraction-8         44.93n ±  1%   29.91n ±  0%  -33.43% (p=0.000 n=10)
FormatFloat/32Point-8                 43.22n ±  1%   33.82n ±  0%  -21.74% (p=0.000 n=10)
FormatFloat/32Exp-8                   45.91n ±  0%   25.48n ±  0%  -44.50% (p=0.000 n=10)
FormatFloat/32NegExp-8                44.66n ±  0%   25.12n ±  0%  -43.76% (p=0.000 n=10)
FormatFloat/32Shortest-8              37.96n ±  0%   27.83n ±  1%  -26.68% (p=0.000 n=10)
FormatFloat/Slowpath64-8              47.74n ±  2%   45.85n ±  0%   -3.96% (p=0.000 n=10)
FormatFloat/SlowpathDenormal64-8      42.78n ±  1%   41.46n ±  0%   -3.07% (p=0.000 n=10)
FormatFloat/ShorterIntervalCase32-8                  25.49n ±  2%
FormatFloat/ShorterIntervalCase64-8                  27.72n ±  1%
geomean                               44.01n         37.10n        -13.52%

Fixes #74886

Author: taichimaeda