add more methods to Assets for clearing and allocation reduction (#669)

crates/bevy_asset/src/assets.rs

@@ -104,6 +104,28 @@ impl<T: Resource> Assets<T> {
         self.assets.remove(&handle)
     }
 
+    /// Clears the inner asset map, removing all key-value pairs.
+    ///
+    /// Keeps the allocated memory for reuse.
+    pub fn clear(&mut self) {
+        self.assets.clear()
+    }
+
+    /// Reserves capacity for at least additional more elements to be inserted into the assets.
+    ///
+    /// The collection may reserve more space to avoid frequent reallocations.
+    pub fn reserve(&mut self, additional: usize) {
+        self.assets.reserve(additional)
+    }
+
+    /// Shrinks the capacity of the asset map as much as possible.
+    ///
+    /// It will drop down as much as possible while maintaining the internal rules and possibly
+    /// leaving some space in accordance with the resize policy.
+    pub fn shrink_to_fit(&mut self) {
+        self.assets.shrink_to_fit()
+    }
+
     pub fn asset_event_system(
         mut events: ResMut<Events<AssetEvent<T>>>,
         mut assets: ResMut<Assets<T>>,