Add Enumerator#cycle.size and Array#cycle.size

More MRI ports for #980.  Compare with [`enum_cycle_size`](https://github.com/ruby/ruby/blob/b7c6db94ae2b9532640609163099c738decb31e6/enum.c#L2291) and [`rb_ary_cycle_size`](https://github.com/ruby/ruby/blob/b7c6db94ae2b9532640609163099c738decb31e6/array.c#L4587).