I think this works

deserialize.rs

pub fn deserialize(bytes: &[u8]) -> Result<Self, AlchemistError> {
    let key_length = bytestou32(&bytes[0..4]) as usize;
    let value_length = bytestou32(&bytes[4..8]) as usize;
    let maybe_key = String::from_utf8(bytes[8 .. 8 + key_length].to_vec());
    let maybe_value = String::from_utf8(bytes[8 + key_length .. 8 + key_length + value_length].to_vec())
    match (maybe_key, maybe_value) {
        (Ok(ref key), Ok(ref value)) => Ok(Self::new(key, value)),
        _ => Err(AlchemistError::DeserializationFailed)
    }
}