Create a Ruby class named "DistinctArray". A DistinctArray is very similar to an Array except for two differences:
    1) it does not contain duplicate elements
    2) it has a method named "to_multiset" which accepts one argument.
  The to_multiset method accepts an argument n. The return value should be an array of many distinct n-length arrays. Each n-length array should contain any number of copies of elements from the parent DistinctArray object.

  Example:
    distinct_array = DistinctArray.new
    3.times {|i| distinct_array.push(i) }
    distinct_array.to_multiset(2) # => [[0,0], [1,2], [1,1], [0,2], [0,1], [2,2]]