Fix unused clippy lint

The `opcodes` module is only used by script which is feature gated on
`alloc`. This causes an unused lint error on `no-default-features`.

Feature gate the whole module on `alloc`.

Author: jamillambert

primitives/src/lib.rs

@@ -41,6 +41,7 @@ pub mod _export {
 }
 
 mod hash_types;
+#[cfg(feature = "alloc")]
 mod opcodes;
 
 pub mod block;