Implement Hash#map_keys, #map_keys!, #map_values, #map_values!, #map_pairs and #map_pairs!

hash_map.rb

class Hash
  def map_keys
    map_pairs { |key, value| [yield(key), value] }
  end
 
  def map_keys!
    map_pairs! { |key, value| [yield(key), value] }
  end
 
  def map_values
    map_pairs { |key, value| [key, yield(value)] }
  end
 
  def map_values!
    map_pairs! { |key, value| [key, yield(value)] }
  end
 
  def map_pairs(&block)
    Hash[map(&block)]
  end
 
  def map_pairs!(&block)
    replace(map_pairs(&block))
  end
end