Make log() use the FmtString infrastructure.

Now `log()` supports `std::string`.

We have to fix a few places where the format parameter was not a compile time constant.
This is mostly trivial.

kernel/io.h

@@ -423,6 +423,7 @@ public:
 	{
 		return format_emit_toplevel(fmt, has_escapes, specs, args...);
 	}
+	std::string_view format_string() const { return fmt; }
 private:
 	std::string_view fmt;
 	bool has_escapes = false;