Use SmallVec in Hash map stable hashing

Author: Kobzol

Diff for: compiler/rustc_data_structures/src/stable_hasher.rs

@@ -552,7 +552,8 @@ pub fn hash_stable_hashmap<HCX, K, V, R, SK, F>(
     SK: HashStable<HCX> + Ord,
     F: Fn(&K, &HCX) -> SK,
 {
-    let mut entries: Vec<_> = map.iter().map(|(k, v)| (to_stable_hash_key(k, hcx), v)).collect();
+    let mut entries: SmallVec<[_; 3]> =
+        map.iter().map(|(k, v)| (to_stable_hash_key(k, hcx), v)).collect();
     entries.sort_unstable_by(|&(ref sk1, _), &(ref sk2, _)| sk1.cmp(sk2));
     entries.hash_stable(hcx, hasher);
 }