Add map_keys and map_values functions

This PR adds two functions that already exist in Hive: the ``map_keys`` and ``map_values`` functions. 

``map_keys(map<K,V>)`` returns the list of keys of the given map as an ``array<K>``

``map_values(map<K,V>)`` returns the list of values of the given map as an ``array<V>``

use a private static variable, since this doesn't change