XCM: Land XCM Builder (#1793)

* Land XCM Builder

* Clean up Cargo dependencies

Co-authored-by: Gavin Wood <gavin@parity.io>

polkadot/parachain/src/primitives.rs

@@ -124,9 +124,21 @@ impl Id {
 	}
 
 	/// Returns `true` if this parachain runs with system-level privileges.
+	/// Use IsSystem instead.
+	#[deprecated]
 	pub fn is_system(&self) -> bool { self.0 < USER_INDEX_START }
 }
 
+pub trait IsSystem {
+	fn is_system(&self) -> bool;
+}
+
+impl IsSystem for Id {
+	fn is_system(&self) -> bool {
+		self.0 < USER_INDEX_START
+	}
+}
+
 impl sp_std::ops::Add<u32> for Id {
 	type Output = Self;