Provides the ruby-1.9 block form of Enumerator, where you can write:

   obj = Enumerator.new do |yielder|
     # ...
     yielder.yield(data)  # or: yielder << data
     # ...
   end

When obj.each is called, the block is run once. It should call yielder.yield with each item it wishes to generate.

Example:

  fib = Enumerator.new { |y|
    a = b = 1
    loop {
      y << a
      a, b = b, a + b
    }
  }

  fib.take(10)  #=> [1, 1, 2, 3, 5, 8, 13, 21, 34, 55]