validation.h: stold => std::from_chars

[RagnarGrootKoerkamp]

@mpsijm @mzuenni what do you think?

https://en.cppreference.com/w/cpp/utility/from_chars
vs
https://en.cppreference.com/w/cpp/string/basic_string/stof

[mpsijm]

This looks like the same fix as #223 (comment), but for long double instead of long long, so I guess that's fine 😄

The CI failure seems to be related to checktestdata, not the validator. I've tested it on the problem set of BAPC 2022, and it seems to still work! 🚀

[mzuenni]

This is somewhat dangerous. Even though from_chars for doubles is part of c++17, compilers only started to support this recently, see compiler support ):
Gcc started to support this with version 11 and it looks like clang with libc++ does not support this at all.

But from_chars is preferable as soon as people stop using gcc 10.

[RagnarGrootKoerkamp]

Ah good find. In that case let's wait.