User defined function to generate a hash code for a given key.

Additionally, for ordered dictionaries (coming soon):

• Your key_hcode mapping must be an invariant mapping from keys to the integer range [-2^31, 2^31). Any predictable structure in the mapping will be removed by the dictionary (i.e. - it is OK if your mapping exhibits strong structure; for example such as mapping all keys of a certain type only to even integers).
• For best performance, a random key should have approximately 1 / 2^32 probability of mapping to any given value.
• Your key_cmp relation and key_hcode mapping must be equality compatible: [key_cmp(key1, key2) == 0 => key_hcode(key1) == key_hcode(key2)]. Note that the reverse implication need not be true.