Expose function directly in transition

lighthouse/state/transition/mod.rs

@@ -1,6 +1,8 @@
 use super::super::utils::types::Hash256;
 
-pub mod attestation_parent_hashes;
+mod attestation_parent_hashes;
+
+pub use self::attestation_parent_hashes::attestation_parent_hashes;
 
 #[derive(Debug)]
 pub enum TransitionError {