This squashes nasty narrowing conversion warnings.

The sheer presence of 'std::gmtime' or 'std::localtime' gives rise to C4996 warnings in Microsofts compilers. Alas, the 'fallback(internal::null<>)'functions containing these library calls are never ADL-picked in the respective 'handle(internal::null<>)' selectors. Therefore hiding the fallbacks from msvc is perfectly fine.

* add make_printf_args and make_wprintf_args to printf.h
add minimal test for make_printf_args and make_wprintf_args to printf-test.cc

renamed the old printf_context template to basic_printf_context_t. the original wish was to rename it basic_printf_context, but that clashed with the name of the inner typedef. this style matches the format_context_t struct.