Document the complexity of operations about golang-set

[deckarep]

It is not O(n), it is only iterating through the arguments you are checking. If you look at the implementation it's doing a direct lookup using a map. This means if you do a Contains of one argument, you'll generally get the same exact runtime complexity as a map. Perhaps either O(1) or O(log n) for more complicated work loads.

[deckarep]

Documenting the runtime is a good idea but it's not something I have time to fully sit down and analyze but I'd welcome a PR.

These implementations are backed by maps so in general you will observe similar performance characteristics. In the more complex operations I've insured to take the obvious optimizations so as to cut down on unnecessary work.