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-struct/src/struct.c

@@ -604,8 +604,8 @@ mrb_struct_eql(mrb_state *mrb, mrb_value s)
 
 /*
  * call-seq:
- *    struct.length   -> Fixnum
- *    struct.size     -> Fixnum
+ *    struct.length   -> Integer
+ *    struct.size     -> Integer
  *
  * Returns number of struct members.
  */