message: add comment explaining EMPTY_HASH_BYTES
diff: @@ -6,7 +6,8 @@ use self::openssl::hash::{Hasher, MessageDigest};
  pub const HASHBYTES: usize = 32;
  -pub const EMPTY_HASH_BYTES : [u8; 32] = [227, 176, 196, 66, 152, 252, 28, 20, 154, 251, 244, 200, 153, 111, 185, 36, 39, 174, 65, 228, 100, 155, 147, 76, 164, 149, 153, 27, 120, 82, 184, 85];
  +// these bytes are the same as openssl_hash(MessageDigest::sha256(), &[]) so we do not have to actually call the hash function
  +pub const EMPTY_HASH_BYTES : [u8; HASHBYTES] = [227, 176, 196, 66, 152, 252, 28, 20, 154, 251, 244, 200, 153, 111, 185, 36, 39, 174, 65, 228, 100, 155, 147, 76, 164, 149, 153, 27, 120, 82, 184, 85];
  pub fn hash(input: &[u8]) -> Result<Vec<u8>, IndyError> {
  let mut hasher = Hash::new_context()?;
  