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)