Use fixed seeds with ahash

src/collections.rs

@@ -0,0 +1,10 @@
+use ahash::AHasher;
+use std::hash::BuildHasherDefault;
+
+/// DOS attacks aren't a concern for Rustlings. Therefore, we use `ahash` with fixed seeds.
+pub type HashSet<T> = std::collections::HashSet<T, BuildHasherDefault<AHasher>>;
+
+#[inline]
+pub fn hash_set_with_capacity<T>(capacity: usize) -> HashSet<T> {
+    HashSet::with_capacity_and_hasher(capacity, BuildHasherDefault::<AHasher>::default())
+}