add get_subtree_index for proofs

ssz-rs/src/merkleization/proofs.rs

@@ -1,6 +1,13 @@
-use crate::merkleization::{MerkleizationError as Error, Node};
+use crate::merkleization::{
+    generalized_index::log_2, GeneralizedIndex, MerkleizationError as Error, Node,
+};
 use sha2::{Digest, Sha256};
 
+pub fn get_subtree_index(i: GeneralizedIndex) -> Result<usize, Error> {
+    let i_log2 = log_2(i).ok_or(Error::InvalidGeneralizedIndex)?;
+    Ok(i % 2usize.pow(i_log2))
+}
+
 /// `is_valid_merkle_branch` verifies the Merkle proof
 /// against the `root` given the other metadata.
 pub fn is_valid_merkle_branch<T: AsRef<[u8]>>(