Simple union-find structure

Simple union-find structure

Implement a simple data structure that, given a list of items
(represented by integer indices), keeps track of a partitioning of these
items into disjoint sets.  Operations to merge two such sets (given two
member items) and to query the "index" of a set are amortized constant
time.  This is a well-known "Union-Find" data structure.

Change-Id: I0bd907755d709776eac416332a4a1bcd139f135f