Array#repeated_permutation should have arity 1

Remove arg-less Array#repeated_permutation to match MRI behavior.  (Noticed this implementing `Array#repeated_permutation.size`: it relies on `#repeated_permutation` having an arg)

```bash
$ ruby -e "p [].repeated_permutation"
-e:1:in `repeated_permutation': wrong number of arguments (0 for 1) (ArgumentError)
	from -e:1:in `<main>'
```

vs.

```bash
$ jruby -e "p [].repeated_permutation"
#<Enumerator: []:repeated_permutation>
```