acc: Do not restrict acceptance codes to small vectors.

* src/twa/acc.hh (acc_word): This is a 32-bit structure, so
we can use short for both size and op.

src/twa/acc.hh

@@ -252,14 +252,14 @@ namespace spot
     };
 
     // This encodes either an operator or set of acceptance sets.
-    enum class acc_op : unsigned char
+    enum class acc_op : unsigned short
     { Inf, Fin, InfNeg, FinNeg, And, Or };
     union acc_word
     {
       mark_t mark;
       struct {
 	acc_op op;	     // Operator
-	unsigned char size; // Size of the subtree (number of acc_word),
+	unsigned short size; // Size of the subtree (number of acc_word),
 			     // not counting this node.
       };
     };