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/lexer.hpp

@@ -84,6 +84,7 @@ struct LexerState {
 	bool atLineStart;
 	uint32_t lineNo;
 	int lastToken;
+	int nextToken;
 
 	std::deque<IfStackEntry> ifStack;