Allow 'dw' and 'dl' to apply to characters of strings

Fixes #568

The old behavior of `dw "string"` can be replicated with `dw ("string")`; likewise for dl

test/asm/db-dw-dl-string.asm

@@ -0,0 +1,17 @@
+SECTION "Test", ROM0
+
+	db "ABC"
+	dw "ABC"
+	dl "ABC"
+
+	db 0, "DEF", -1
+	dw 0, "DEF", -1
+	dl 0, "DEF", -1
+
+	db "A" + 1
+	dw "A" + 1
+	dl "A" + 1
+
+	db 1, ("UVWXYZ") & $ff, -1
+	dw 1, ("UVWXYZ") & $ffff, -1
+	dl 1, ("UVWXYZ"), -1