Add UnionFind data structure with a Rust backend.

[aalekhpatel07]

Feature:

• A fast implementation of the UnionFind data structure that has a Rust backend.

Minor updates:

• Refactor the Solution class in algorithms/unionfind/count_islands.py to use the new UnionFind API.

Bug Fixes:

• Add None checks at the boundaries for Array limit (so that the tests pass).