def map!(&block)
      (0...length).each do |i|
        put(i, block.call(get(i)))
      end
      self
    end
def map!(&block)
      (0...length).each do |i|
        put(i, block.call(get(i)))
      end
      self
    end
Map each element and store the result in the matrix.

 Note that the result must be again something
 which can be stored in the matrix. Otherwise you should do an
 to_a first.