Common set-theory comparisons and operators

The Set API in javascript is currently rather impoverished, with the ability to define sets but little in the way of common set-theory operations or comparisons we can easily perform on them.

This module exposes a number of useful set-theory operations and comparisons:

* union(a, b): Return everything in set A or B
* intersection(a, b): Return everything in set A and B
* difference(a, b): Subtract from set A everything in B
disjunctiveUnion(a, b): Return everything in set A xor B (everything in set A or B but not both*)
* equal(a, b): Returns true if two sets are equal (sets have no concept of ordering, to this relies purely on the presence/absence of values, not their order)
isSubsetOf(a, b): Returns true if A is a subset of B (equal sets are* subsets of each other)
isStrictSubsetOf(a, b): Returns true if A is a strict subset of B (equal sets are not* strict subsets of each other)
isSupersetOf(a, b): Returns true if A is a superset of B (equal sets are* supersets of each other)
isStrictSupersetOf(a, b): Returns true if A is a strict superset of B (equal sets are not* strict supersets of each other)