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-load.asm

@@ -0,0 +1,14 @@
+SECTION "OAMDMACode", ROM0
+OAMDMACode:
+LOAD "hOAMDMA", HRAM
+hOAMDMA::
+	ldh [$ff46], a
+	ld a, 40
+	jp [[
+:		dec a
+		jr nz, :-
+		ret
+	]]
+.end
+ENDL
+OAMDMACodeEnd: