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.

include/mruby/class.h

@@ -36,7 +36,7 @@ mrb_class(mrb_state *mrb, mrb_value v)
   case MRB_TT_SYMBOL:
     return mrb->symbol_class;
   case MRB_TT_FIXNUM:
-    return mrb->fixnum_class;
+    return mrb->integer_class;
 #ifndef MRB_NO_FLOAT
   case MRB_TT_FLOAT:
     return mrb->float_class;