[meta] Add a MapWithDefault trait;

This traits augments HashMap so they have a `get_or_default` method that
can be used to avoid boilerplate around usage of `entry.or_default`.

cranelift/codegen/meta/src/default_map.rs

@@ -0,0 +1,19 @@
+use std::collections::HashMap;
+use std::hash::Hash;
+
+pub trait MapWithDefault<K, V: Default> {
+    fn get_or_default(&mut self, k: K) -> &mut V;
+}
+
+impl<K: Eq + Hash, V: Default> MapWithDefault<K, V> for HashMap<K, V> {
+    fn get_or_default(&mut self, k: K) -> &mut V {
+        self.entry(k).or_insert_with(|| V::default())
+    }
+}
+
+#[test]
+fn test_default() {
+    let mut hash_map = HashMap::new();
+    hash_map.insert(42, "hello");
+    assert_eq!(*hash_map.get_or_default(43), "");
+}