Remove dependency of variant.h in print_string.h

[YYF233333]

print_string.h isn’t the lightweight header its name suggests—concerned only with String—but a fairly heavy one, because it includes variant.h internally. The only place it actually needs Variant is in stringfy_variants(). I changed that function to a non-template version and moved it into print_string.cpp, so now print_string.h only depends on ustring.h.

std::initializer_list may incur some overhead, but these functions don't seem to be particularly performance-sensitive, so I opted for the most straightforward implementation.

[Ivorforce]

This is definitely worth doing.

But instead of std::initializer_list, it should use Span, I think. initializer_list is more meant for actual initialization, so it feels out-of-domain here.

You should also be able to simplify to just one templated function, by using something like this:

Variant variants[sizeof...(args)] = { args... };
__print_line(stringify_variants(Span(variants))

[YYF233333]

But instead of std::initializer_list, it should use Span, I think. initializer_list is more meant for actual initialization, so it feels out-of-domain here.

You should also be able to simplify to just one templated function, by using something like this:

Changed and simplified. There is one more copy for one argument situation, but the overhead should be negiligible (actually I think compiler can optimize this away).

[Ivorforce]

Works for me.