Cykhash is a cython equivalent to khash-sets/maps, efficient
implementation of isin and unique

Benefits:
    Brings functionality of khash to Python and Cython and can be used
    seamlessly in numpy or pandas.
    Numpy's world is lacking the concept of a (hash-)set. This
    shortcoming is fixed and efficient (memory- and speedwise compared
    to pandas') unique and isin are implemented.
    Python-set/dict have big memory-footprint. For some datatypes the
    overhead can be reduced by using khash by factor 4-8.