Remove the obsolete term Fixnum.

Except for compatibility code.

src/object.c

@@ -409,7 +409,7 @@ mrb_check_type(mrb_state *mrb, mrb_value x, enum mrb_vtype t)
     ename = "nil";
   }
   else if (mrb_integer_p(x)) {
-    ename = "Fixnum";
+    ename = "Integer";
   }
   else if (mrb_symbol_p(x)) {
     ename = "Symbol";