Add anonymous labels

Fix #497

test/asm/anon-label-bad.asm

@@ -0,0 +1,18 @@
+
+: ; Outside of section
+
+SECTION "Anonymous label errors test", ROM0
+
+	db :-- ; Reference goes too far back
+
+; Uncomment this if you're a badass with a *lot* of RAM
+; REPT 2147483647
+; :
+; ENDR
+; REPT 2147483647
+; :
+; ENDR
+; 	db :+ ; OK
+; 	db :++ ; Reference goes too far
+
+:: ; Syntax error, can't export this

test/asm/anon-label-bad.err

@@ -0,0 +1,7 @@
+ERROR: anon-label-bad.asm(2):
+    Label "!0" created outside of a SECTION
+ERROR: anon-label-bad.asm(6):
+    Reference to anonymous label 2 before, when only 1 has been created so far
+ERROR: anon-label-bad.asm(18):
+    syntax error
+error: Assembly aborted (3 errors)!

test/asm/anon-label.asm

@@ -0,0 +1,12 @@
+
+SECTION "Anonymous label test", ROM0[0]
+
+	ld hl, :++
+:	ld a, [hli]
+	ldh [c], a
+	dec c
+	jr nz, :-
+	ret
+
+:
+	dw $7FFF, $1061, $03E0, $58A5