feat: Add advanced set operations

[taylorsw04]

This PR adds advanced set operations to the extended/ library:

• Union (merge two trees)
• Intersect (keys present in both trees)
• Subtract (keys in a tree that are not present in another tree)
  Note: symmetric difference (merge minus intersection) can be computed via union + combineFn that excludes all shared keys.

Each of the above methods returns a new tree. To avoid the allocation overhead (e.g. if the result is only to be enumerated), iteration/callback flavors of each have been added (intersect -> forEachKeyInBoth, subtract -> forEachKeyNotIn).

Finally, a bulk loading algorithm has been exposed allowing consumers to build a BTree from a sorted list of entries in O(N) time.

All of the above has been added to BTreeEx as well as exposed via standalone methods for optimal tree-shaking.

Size report:

Entry Raw Size Minified Gzipped
btree 71.43 KB 19.49 KB 5.50 KB
extended/bulkLoad 4.96 KB 1.95 KB 932 B
extended/decompose 34.15 KB 8.01 KB 2.80 KB
extended/diffAgainst 12.75 KB 2.47 KB 1.05 KB
extended/forEachKeyInBoth 3.59 KB 1.09 KB 455 B
extended/forEachKeyNotIn 4.25 KB 1.23 KB 484 B
extended/index 11.02 KB 2.78 KB 987 B
extended/intersect 2.06 KB 732 B 403 B
extended/parallelWalk 6.59 KB 1.92 KB 935 B
extended/shared 4.28 KB 1.60 KB 606 B
extended/subtract 2.14 KB 532 B 328 B
extended/union 1.92 KB 472 B 294 B
BTreeEx total 87.71 KB 22.75 KB 9.15 KB