Implement the ShrinkIndex algebra

At the very least, we will want:

```
data ShrinkIndex
data ShrinkTree a

makeShrinkTree :: (a -> [a]) -> a -> ShrinkTree a
lookup :: ShrinkIndex -> ShrinkTree a -> Maybe a
```

plus the extend and succ operations discussed previously.

Ideally tests will be provided showing that these operations behave as we expect them to.