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>``

you don't need the @SqlType annotation here, since map_keys and map_values are ParametricFunctions that bind the MethodHandle. @SqlType is only needed if you're relying on the non-parametric function annotation parsing logic