Add hb_pair_t(,) macro as alternative to hb_pair_t<,>

Just so it's easier to use it in other macros.

src/hb-algs.hh

@@ -222,6 +222,7 @@ struct hb_pair_t
   T1 first;
   T2 second;
 };
+#define hb_pair_t(T1,T2) hb_pair_t<T1, T2>
 template <typename T1, typename T2> static inline hb_pair_t<T1, T2>
 hb_pair (T1&& a, T2&& b) { return hb_pair_t<T1, T2> (a, b); }