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.
Benchmarks and fuzz-testing code are available at:
https://github.com/taichimaeda/dragonbox-bench/

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

Fixes #74886

Author: taichimaeda