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/code-after-endm-endr-endc.err

@@ -1,15 +1,15 @@
 error: code-after-endm-endr-endc.asm(6):
-    syntax error, unexpected PRINTLN, expecting end of line or end of buffer
+    syntax error, unexpected PRINTLN, expecting end of line or end of buffer or end of fragment literal
 error: code-after-endm-endr-endc.asm(7):
     Macro "mac" not defined
 error: code-after-endm-endr-endc.asm(12):
-    syntax error, unexpected PRINTLN, expecting end of line or end of buffer
+    syntax error, unexpected PRINTLN, expecting end of line or end of buffer or end of fragment literal
 error: code-after-endm-endr-endc.asm(17):
     syntax error, unexpected PRINTLN, expecting end of line
 error: code-after-endm-endr-endc.asm(19):
-    syntax error, unexpected PRINTLN, expecting end of line or end of buffer
+    syntax error, unexpected PRINTLN, expecting end of line or end of buffer or end of fragment literal
 error: code-after-endm-endr-endc.asm(23):
     syntax error, unexpected PRINTLN, expecting end of line
 error: code-after-endm-endr-endc.asm(25):
-    syntax error, unexpected PRINTLN, expecting end of line or end of buffer
+    syntax error, unexpected PRINTLN, expecting end of line or end of buffer or end of fragment literal
 Assembly aborted with 7 errors!

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:

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

@@ -0,0 +1,2 @@
+FATAL: fragment-literal-in-load.asm(7):
+    `LOAD` blocks cannot contain 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"
+]]

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

@@ -0,0 +1,2 @@
+FATAL: fragment-literal-in-ram.asm(6):
+    Section 'RAM' cannot contain fragment literals (not ROM0 or ROMX)

test/asm/fragment-literal-in-ram.out

@@ -0,0 +1 @@
+ok

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

@@ -0,0 +1,5 @@
+SECTION UNION "U", ROM0
+	db $11
+	dw [[ db $22 ]]
+SECTION UNION "U", ROM0
+	db $33

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

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

test/asm/syntax-error-after-syntax-error.err

@@ -7,5 +7,5 @@ error: syntax-error-after-syntax-error.asm(6):
 error: syntax-error-after-syntax-error.asm(9):
     syntax error, unexpected :
 error: syntax-error-after-syntax-error.asm(10):
-    syntax error, unexpected stop, expecting end of line or end of buffer or ::
+    syntax error, unexpected stop, expecting end of line or end of buffer or end of fragment literal or ::
 Assembly aborted with 5 errors!