impl PartialOrd via is_{sub,super}set

[ronnodas]

Instead of a.is_subset(b), we should be able to write a <= b.

[coriolinus]

An interesting proposal! My first reaction is positive, though I haven't yet devoted any real thought to whether this would cause any problems.

Please go ahead and write an implementation PR; while I kind of like this idea, it's very low on my personal priority list.