XCM: Allow reclaim of assets dropped from holding (#3727)

* XCM: Introduce AssetTrap

* Revert reversions

* Remove attempts at weighing and add test

* Less storage use for asset trapping

* Add missing file

* Fixes

* Fixes

* Formatting

* Fixes

* Docs

* Filter types to allow runtimes to dictate which assets/origins should be trapped

* Formatting

* Tests

* Formatting

* Fixes

* Docs

polkadot/xcm/xcm-executor/src/assets.rs

@@ -94,6 +94,11 @@ impl Assets {
 		self.fungible.len() + self.non_fungible.len()
 	}
 
+	/// Returns `true` if `self` contains no assets.
+	pub fn is_empty(&self) -> bool {
+		self.fungible.is_empty() && self.non_fungible.is_empty()
+	}
+
 	/// A borrowing iterator over the fungible assets.
 	pub fn fungible_assets_iter<'a>(&'a self) -> impl Iterator<Item = MultiAsset> + 'a {
 		self.fungible