Implement [[ fragment literals ]] (#1614)

This feature is referred to as "code/data literals" in ASMotor,
and simply as "literals" in some older assemblers like MIDAS
for the PDP-10. RGBASM already had the "section fragments"
feature for keeping disparate contents together when linked,
so these worked naturally as "fragment literals".

include/asm/section.hpp

@@ -104,4 +104,6 @@ void sect_PushSection();
 void sect_PopSection();
 void sect_CheckStack();
 
+std::string sect_PushSectionFragmentLiteral();
+
 #endif // RGBDS_ASM_SECTION_HPP