Add support for 'std::variant' in C++17.
For C++17, if all the alternatives of a variant are formattable
the variant is now also formattable. In addition 'std::monostate'
is now formattable.
Moves implementation into 'std.h', and tests into 'std-test.cc'.
Avoid fold-expression since MSVC was crashing.
Add section for 'fmt/std.h' in API-docs.