Should have been using if-let all along, not a list of pairs of nil. This makes for an easy distinction between the nil initialization value and using nil as a key in the map: one is a list and the other is just a nil value.
Renamed en route to making a test file. I guess that's the right way to do this. Plus, I can get it to run automatically in LT which is cool.
Proper unit tests. Mutual recursion?

Are `hash-put` and `resize` really going to have to be mutually
recursive? Jeese, that certainly seems like a bit extreme.