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".

test/asm/fragment-literal-in-union.err

@@ -0,0 +1,2 @@
+FATAL: fragment-literal-in-union.asm(3):
+    `SECTION UNION` cannot contain fragment literals