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