Implement ht find

Solves #61 

Note: I assume `s` from the paper being the size (number of buckets) of the hashtable.
Declaring an iterator before the loop and then having a 0-check as the condition seem odd.
Imho the following would be more elegant:
```c
while (--i) {
    avl_destroy(&ht->buckets[i].avl)
}
```