Add / remove from group + clean up code.

src/util/mod.rs

@@ -1,6 +1,6 @@
 use hashbrown::HashSet;
 use std::iter::FromIterator;
 
-pub fn vec_to_set<T: Clone + Eq + std::hash::Hash>(data: &Vec<T>) -> HashSet<T> {
+pub fn vec_to_set<T: Clone + Eq + std::hash::Hash>(data: &[T]) -> HashSet<T> {
     HashSet::from_iter(data.iter().cloned())
 }