Check in some tests showing that there is some sort of bug in jwHash.c. ...

I know you're aware `add_entry` collision handling isn't finished yet. Here are two programs that illustrate the bug.

benchmark.py and benchmark.c are the same algorithm, but if you run them both, they'll give different answers.

    ~/dev/c-hashtable$ python3 benchmark.py < sam.txt
    sam
    i
    ...
    Found 51 different words.
    
    ~/dev/c-hashtable$ make benchmark && ./benchmark < sam.txt
    g++ -O3 -o benchmark benchmark.c
    sam
    add hash: 48
    i
    add hash: 14
    ...
    Found 137 different words.

(Python's `set` object is a hash table. The code that says `if word not in seen` is basically doing `look_up(word)`. The code that says `seen.add(word)` is doing `add_entry("", "", word)`.)