Add missing includes to type.h (#1491)

- cassert for assert
- confing.h for WABT_UNREACHABLE

I think it was working because type.h was only included in common.h,
both cassert and config.h was included prior to including type.h.

Author: ngzhian

src/type.h

@@ -17,9 +17,12 @@
 #ifndef WABT_TYPE_H_
 #define WABT_TYPE_H_
 
+#include <cassert>
 #include <cstdint>
 #include <vector>
 
+#include "config.h"
+
 namespace wabt {
 
 class Type;