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, excluding
fixed-precision cases. Full benchmark results and plots are available at:
https://github.com/taichimaeda/dragonbox-bench/

goos: darwin
goarch: arm64
pkg: strconv
cpu: Apple M1

Ryu:

BenchmarkFormatFloat/Decimal-8         	36197865	        31.45 ns/op	       0 B/op	       0 allocs/op
BenchmarkFormatFloat/Float-8           	26408643	        45.49 ns/op	       0 B/op	       0 allocs/op
BenchmarkFormatFloat/Exp-8             	23845756	        49.87 ns/op	       0 B/op	       0 allocs/op
BenchmarkFormatFloat/NegExp-8          	25607784	        47.04 ns/op	       0 B/op	       0 allocs/op
BenchmarkFormatFloat/LongExp-8         	26079632	        46.16 ns/op	       0 B/op	       0 allocs/op
BenchmarkFormatFloat/Big-8             	24082401	        49.99 ns/op	       0 B/op	       0 allocs/op
BenchmarkFormatFloat/BinaryExp-8       	42700762	        27.90 ns/op	       0 B/op	       0 allocs/op
BenchmarkFormatFloat/32Integer-8       	38270085	        31.41 ns/op	       0 B/op	       0 allocs/op
BenchmarkFormatFloat/32ExactFraction-8 	26591202	        45.31 ns/op	       0 B/op	       0 allocs/op
BenchmarkFormatFloat/32Point-8         	27870975	        43.20 ns/op	       0 B/op	       0 allocs/op
BenchmarkFormatFloat/32Exp-8           	26127472	        45.93 ns/op	       0 B/op	       0 allocs/op
BenchmarkFormatFloat/32NegExp-8        	26843410	        44.70 ns/op	       0 B/op	       0 allocs/op
BenchmarkFormatFloat/32Shortest-8      	31629616	        37.95 ns/op	       0 B/op	       0 allocs/op
BenchmarkFormatFloat/Slowpath64-8      	25230336	        47.78 ns/op	       0 B/op	       0 allocs/op
BenchmarkFormatFloat/SlowpathDenormal64-8         	27206694	        43.22 ns/op	       0 B/op	       0 allocs/op

Dragonbox:

BenchmarkFormatFloat/Decimal-8         	39320097	        30.38 ns/op	       0 B/op	       0 allocs/op
BenchmarkFormatFloat/Float-8           	28416686	        42.02 ns/op	       0 B/op	       0 allocs/op
BenchmarkFormatFloat/Exp-8             	37673996	        31.86 ns/op	       0 B/op	       0 allocs/op
BenchmarkFormatFloat/NegExp-8          	38569448	        31.11 ns/op	       0 B/op	       0 allocs/op
BenchmarkFormatFloat/LongExp-8         	27570123	        43.34 ns/op	       0 B/op	       0 allocs/op
BenchmarkFormatFloat/Big-8             	30569094	        38.95 ns/op	       0 B/op	       0 allocs/op
BenchmarkFormatFloat/BinaryExp-8       	43578852	        28.05 ns/op	       0 B/op	       0 allocs/op
BenchmarkFormatFloat/32Integer-8       	52877412	        22.81 ns/op	       0 B/op	       0 allocs/op
BenchmarkFormatFloat/32ExactFraction-8 	40459780	        29.81 ns/op	       0 B/op	       0 allocs/op
BenchmarkFormatFloat/32Point-8         	36037794	        33.68 ns/op	       0 B/op	       0 allocs/op
BenchmarkFormatFloat/32Exp-8           	47398902	        25.32 ns/op	       0 B/op	       0 allocs/op
BenchmarkFormatFloat/32NegExp-8        	47989363	        25.04 ns/op	       0 B/op	       0 allocs/op
BenchmarkFormatFloat/32Shortest-8      	43502100	        27.76 ns/op	       0 B/op	       0 allocs/op
BenchmarkFormatFloat/Slowpath64-8      	26418043	        45.49 ns/op	       0 B/op	       0 allocs/op
BenchmarkFormatFloat/SlowpathDenormal64-8         	28887122	        41.43 ns/op	       0 B/op	       0 allocs/op
BenchmarkFormatFloat/ShorterIntervalCase32-8      	47092680	        25.95 ns/op	       0 B/op	       0 allocs/op
BenchmarkFormatFloat/ShorterIntervalCase64-8      	42481884	        27.65 ns/op	       0 B/op	       0 allocs/op

Fixes #74886

Author: taichimaeda