<charconv>: Sync with upstream Ryu

xcharconv_ryu.h is designed to stay in sync with upstream Ryu, so we can benefit from @ulfjack's continuing improvements and contribute changes back when they make sense.

The last commit that I synced to was https://github.com/ulfjack/ryu/commit/59661c3f883dfd39cef6dc8eaf2fcbaae53597e8 on 2019-04-02. Since then, there have been a moderate number of changes upstream, including https://github.com/ulfjack/ryu/commit/7cd455c923321d1f483a755d027b486103d9c3ae which notably unified away the tables for shortest float.

At this time, the process for syncing upstream changes is labor-intensive and documented only within my own brain. (It involves https://github.com/StephanTLavavej/ryu/tree/msvc-latest and lots of rebasing.) I need to:

1. Sync with upstream.
2. Update cgmanifest.json and start the associated Microsoft-internal process.
3. Profile the table unification (affects #172).
4. Contribute as many changes as I can upstream. Ulf mentioned that the bounds-checking logic is welcome.
5. Attempt to improve this process (it really shouldn't involve a personal branch, for example).