Add support for 'std::variant' in C++17

[jehelset]

Quick variant implementation based on same machinery as tuple-formatting in #2940.

I just wanted to submit it to see if it was acceptable.

[phprus]

My suggestions:

Move variant.h content into std.h.
Use #ifdef __cpp_lib_variant instad of #if FMT_HAS_VARIANT.

[vitaut]

Thanks for the PR. This should definitely go to fmt/std.h and please fix build failures.

[jehelset]

i rewrote write_range_entry from fmt/range.h as write_variant_alternative. since variant is C++17, i hoped i can could get away with if-constexpr instead, and compilers didn't complain. i just used is_string from fmt/core.h instead of this is_std_string_like in fmt/ranges.h, to not have to include or move code around. i also took the liberty of using std::integral_sequence, std::void_t and std::conjuction, and instead of checking for those library features i used the __cpp_lib_variant as a proxy. that's not quite correct, but i'm not sure what is the best approach.

[vitaut]

Overall looks good but I'd suggest changing the default representation to variant(...). Also a few comments inline.

[jehelset]

Overall looks good but I'd suggest changing the default representation to variant(...). Also a few comments inline.

If we can be verbose I would prefer to print std::monostate as monostate, and a variant holding a monostate as just variant(monostate). Probably could do with some format specifiers, but I will leave that for someone more proficient.

[vitaut]

Thank you!