++ use NULL instead of a sentinal node in skiplist and list
++ make the interfaces for all data structures consistent
++ make list and skiplist use string keys
++ optimize integer keys
++ ht_print()
+- iterators
+- characterize performance of data structures
+- experiment with the performance impact of not passing the hash between functions
+- experiment with embedding keys in the list/skiplist nodes
+- allow values of 0 to be inserted into maps (change DOES_NOT_EXIST to something else)