Implement property-based tests using `hedgehog`

This is a meta-issue to collect all possible laws for the `Treap` data structure:

### Semigroup/Monoid laws

- [x] Associativity
- [x] Left/Right identity

### Measured

- [ ] `measure (one x) ≡ x`

### IsList

- [ ] Roundtrip `IsList`: `toList . fromList ≡ id`

### Modifications

- [ ] `at i . insert i x ≡ Just x`
- [ ] `delete i . insert i x ≡ id`
- [ ] `size (insert i x t) ≡ size t + 1`

### Cuts

- [ ] `uncurry merge . splitAt i ≡ id`
- [ ] `size (merge l r) ≡ size l + size r`
- [ ] Monoid homomorphism: `measure (merge l r) ≡ measure l <> measure r`
- [ ] `splitAt i t ≡ (take i t, drop i t)`

Start by locating the Treap implementation and its existing test entry point, then review the unchecked laws listed in this issue. Use Hedgehog to cover the measured, IsList, modification, and cut laws, and consider the work done when each listed property is implemented and passes.

Written by the indexing model from the issue text.