Add Disjoint Set ADT

[cyderize]

Adds a disjoint set ADT with the following operations:

• disjoint set [items]
  • returns a new disjoint set (with the given items as singletons)
• add x to disjoint set d
  • adds x to the disjoint set as a singleton
• union x and y in disjoint set d
  • joins the subsets containing x and y together
• find x in disjoint set d
  • returns the element that represents which subset x belongs to

Note that the watcher morph for this ADT is based on lists, and is not awfully efficient (but better than not being able to see anything at all).

This could be used for example in Kruskal's algorithm for the MST.

Closes #407.

[stevenbird]

This works nicely, thanks @cyderize