Improve float-to-string conversion using Ryu

[tgregg]

The text writer currently uses sprintf for its float-to-string conversions. Ryu may provide an opportunity to improve both the correctness and performance of this conversion.

From the authors:

We present Ryū, a new routine to convert binary floating point numbers to their decimal representations using only fixed-size integer operations, and prove its correctness. Ryū is simpler and approximately three times faster than the previously fastest implementation.

See also:

• The Github repository, which includes a C implementation.
• A video, which introduces the algorithm.
• The paper, which includes the proof.
• The related ion-java issue.