* add a destructor function for elements
* rename cuckoo_free to cuckoo_delete to be consistent with the rest
* add a custom copy function to the cuckoo hash
* Cuckoo hash update, use a vtable to pass functions to the constructor
* make the source of a move non-const, just in case something needs
to be cleaned up there.
* let cuckoo_insert return the hash table entry directly
This implementation of cuckoo hashing gives guaranteed O(1)
lookup complexity and amortized O(1) insert and remove complexity.
Hash tables by default grow and shrink automatically.
It is posible to preallocate entries and turn automatic shrinking off,
taking out the memory management factors for insert and remove operations.
