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-literals.asm

@@ -0,0 +1,62 @@
+SECTION "1", ROM0[0]
+
+DEF VERSION EQU $11
+GetVersion::
+	ld a, [ [[db VERSION]] ]
+	ret
+
+SECTION "2", ROM0, ALIGN[4]
+
+MACRO text
+	db \1, 0
+ENDM
+
+MACRO text_pointer
+	dw [[
+		text \1
+	]]
+ENDM
+
+GetText::
+	ld hl, [[
+		dw [[ db "Alpha", 0 ]]
+		dw [[
+			text "Beta"
+		]]
+		text_pointer "Gamma"
+		dw 0
+	]]
+	ld c, a
+	ld b, 0
+	add hl, bc
+	add hl, bc
+	ld a, [hli]
+	ld h, [hl]
+	ld l, a
+	ret
+
+SECTION "C", ROM0
+
+Foo::
+	call [[ jp [[ jp [[ ret ]] ]] ]]
+	call [[
+Label::
+		call GetVersion
+		DEF MYTEXT EQU 3
+		ld a, MYTEXT
+		call GetText
+		ld b, h
+		ld c, l
+		ret
+	]]
+	jp [[
+Bar:
+		inc hl
+.loop
+		nop
+:		dec l
+		jr nz, :-
+		dec h
+		jr nz, .loop
+		ret
+	]]