Implement property-based tests using hedgehog

[chshersh]

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

Semigroup/Monoid laws

• Associativity
• 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)