Add make_set! to add a new set with a single element for IntDisjointSets and DisjointSets

Adds `make_set!` to add a new set with a single element for `DisjointSets`. Three versions are available:
  - add an automatically-generated element for `IntDisjointSets`
  - add a specified element for `IntDisjointSets` and `DisjointSets`