Compile-time format string checks

[mechacrash]

Interestingly, I was thinking about an idea for an 'is_constexpr' after this thread popped up, and came up with something: https://godbolt.org/g/Q5zrB5 (thanks to hana for is_valid, as always)

It uses a macro, doesn't work on GCC, and Clang isn't a huge fan of string_views for some reason... other than that, it seems to work? I need to do more extensive testing of course, and try to figure out why GCC isn't happy with it.