disjoint-sets: two union-find implementations

This library provides two disjoint set data structures:

• UnionFind: An array-based union-find where clients represent elements as small unsigned integers.
• UnionFindNode: A tree-based union-find where each set can have associated ata, and where clients represent elements as opaque tree nodes.

Both perform rank-balanced path compression à la Tarjan, using interior mutability.