Integrate Fixnum class into Integer class

* The `Fixnum` constant is now an alias for the `Integer` class.
* Remove `struct mrb_state::fixnum_class` member.
  If necessary, use `struct mrb_state::integer_class` instead.

mrbgems/mruby-socket/src/socket.c

@@ -145,7 +145,7 @@ mrb_addrinfo_getaddrinfo(mrb_state *mrb, mrb_value klass)
   } else if (mrb_nil_p(service)) {
     servname = NULL;
   } else {
-    mrb_raise(mrb, E_TYPE_ERROR, "service must be String, Fixnum, or nil");
+    mrb_raise(mrb, E_TYPE_ERROR, "service must be String, Integer, or nil");
   }
 
   memset(&hints, 0, sizeof(hints));