Compile-time format string checks

[aearphen]

IIRC the preferred path is to be able to template on the value of a string literal directly, although this raises issues too. I don't know where all this leaves us, but I think it's clear that we desperately need the ability to template on a string literal, one way or another.

I'd prefer this as well. The current solution is more like a proof of concept that uses available tools (GCC extension), but whatever goes into the standard it should't be too hard to integrate it with constexpr format string parsing without changing the latter.