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-ram.asm

@@ -0,0 +1,9 @@
+SECTION "RAM", WRAM0
+
+wFoo:: db
+wBar:: ds 3
+	println "ok"
+wQux:: dw [[
+	ds 4
+	println "inline"
+]]