* #2954: Provide std::conjunction and std::disjunction substitutes
* #2954: Use conjunction and disjunction substitute to make formatter specializations for ranges and maps more robust (especially for Visual Studio compiler family)
* #2954: As workaround for older MSVC compilers split formatter<std::filesystem::path> partial template specialization into two explicit specialization.
* 2954: Add test case
* Provide simplified implementations of conjunction and disjunction
* Remove workaround explicit specializations if the partial specialization would cause an ambiguity error
* Eliminate extra-test and merge it into existing std-test instead. Add conditionals for filesystem::path testing that does not run into the ambiguity problem.
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.
