[PATCH] Define std::to_chars and std::from_chars for C++17 (P0067R5, partial)

[Florian Weimer]

* Jonathan Wakely:

> That respects the locale, which std::from_chars explicitly ignores,
> but it's a lot of work nonetheless. I think we want to implement
> Grisu3 and then have something else as a slower fallback for the cases
> Grisu3 fails on.

We'd also want these algorithms in glibc, so that C programmers can
write proper JSON serializers.

(It's silly how many of them produce invalid JSON in certain locales,
and how few can properly round-trip all numbers.)