fix(types): conditionally import context (#774)

Author: grtlr

src/types/mod.rs

@@ -1,6 +1,7 @@
 // Copyright 2022 IOTA Stiftung
 // SPDX-License-Identifier: Apache-2.0
 
+#[cfg(feature = "stardust")]
 pub mod context;
 #[cfg(feature = "stardust")]
 pub mod ledger;