refactor: Introduce struct to hold a runtime format string

This brings the format types closer to the standard library types:

* FormatStringCheck corresponds to std::basic_format_string, with
  compile-time checks done via ConstevalFormatString
* RuntimeFormat corresponds to std::runtime_format, with no compile-time
  checks done.

Also, it documents where no compile-time checks are done.

src/test/util_string_tests.cpp

@@ -16,7 +16,7 @@ template <unsigned NumArgs>
 void TfmFormatZeroes(const std::string& fmt)
 {
     std::apply([&](auto... args) {
-        (void)tfm::format(fmt, args...);
+        (void)tfm::format(tfm::RuntimeFormat{fmt}, args...);
     }, std::array<int, NumArgs>{});
 }