Add count-sets; make count return total #elts

count-sets returns the number of disjoint sets now, whereas count
returns the total number of elements in the structure. This is a bit
more Clojurey, I think.

Potential idea for fixing #2
seq returns all elts, not just canonicals

This is a bit more Clojure-y as well, I think. Getting all of the
canonical elements can be done yourself with a filter.

Potential idea for fixing #2