Add union-find crate

Author: qryxip

Cargo.lock

@@ -133,6 +133,9 @@ im-rc = "=14.0.0"
 fixedbitset = "=0.2.0"
 bitset-fixed = "=0.1.0"
 
+# union-find (a.k.a. disjoint-set)
+union-find = "=0.3.2"
+
 # 競技プログラミングの入出力サポート
 proconio = { version = "=0.3.4", features = ["derive"] }
 text_io = "=0.1.7"