code: def map(self, func):
      
      self._data = xmap(func, self._data)
      return self
docs: A lazy way to apply the given function to each element in the stream.
      Useful for type casting, like:
  
      >>> from audiolazy import count
      >>> count().take(5)
      [0, 1, 2, 3, 4]
      >>> my_stream = count().map(float)
      >>> my_stream.take(5) # A float counter
      [0.0, 1.0, 2.0, 3.0, 4.0]