<charconv>: Investigate the Eisel-Lemire ParseNumberF64 algorithm for from_chars()

• https://lemire.me/blog/2020/03/10/fast-float-parsing-in-practice/
  • Repo: https://github.com/lemire/fast_double_parser (the Boost license is compatible with the STL)
• https://lemire.me/blog/2021/01/29/number-parsing-at-a-gigabyte-per-second/
• https://nigeltao.github.io/blog/2020/eisel-lemire.html
• https://nigeltao.github.io/blog/2020/parse-number-f64-simple.html

The current implementation of from_chars() that I originally shipped in VS 2017 15.8 (released Aug 2018) is a refined version of the UCRT's strtod() with no algorithmic improvements. (At the time I concluded, apparently correctly, that no substantially better algorithms were known, and the UCRT was the only practical option regarding licensing.)

Now that Future Technology 🛰️ is available, we can look into using it. (After finishing C++20!)