Function reverse_key 

pub fn reverse_key(
    namespace: &str,
    edge_type: &str,
    to_id: &RecordId,
    from_id: &RecordId,
) -> Vec<u8> 

Build a reverse index key.

Format: [0x03][ns_hash: 4B][et_hash: 4B][to_key_len: 2B][to_key][from_key_len: 2B][from_key]