Allow NUL characters in strings (#1405)

test/asm/null-outside-string.asm

@@ -0,0 +1,7 @@
+SECTION "test", ROM0
+	; '\0' is not special here; it's lexed as a line continuation...
+	DEF foo\0bar EQU 42
+	db foo\0bar
+	; ...just like any other non-whitespace character
+	DEF spam\Xeggs EQU 69
+	db spam\Xeggs