Add support for some serde serialization (#847)

* Add support for some serde serialization

cranelift/entity/src/map.rs

@@ -56,11 +56,23 @@ where
         }
     }
 
+    /// Returns the number of elements in the underlying vector.
+    ///
+    /// The number is not necessarily the same as the length of the corresponding PrimaryMap.
+    pub fn len(&self) -> usize {
+        self.elems.len()
+    }
+
     /// Get the element at `k` if it exists.
     pub fn get(&self, k: K) -> Option<&V> {
         self.elems.get(k.index())
     }
 
+    /// Get the default value.
+    pub fn get_default(&self) -> &V {
+        &self.default
+    }
+
     /// Is this map completely empty?
     pub fn is_empty(&self) -> bool {
         self.elems.is_empty()