Better empty struct rust code gen

[mattiekat]

Noticed rust was being a bit dumb on empty structs and it was an easy fix.

Before it would generate

fn _deserialize_chained(raw: &'raw [u8]) -> ::bebop::DeResult<(usize, Self)> {
    let mut i = 0;
    if raw.len() - i < Self::MIN_SERIALIZED_SIZE {
        let missing = Self::MIN_SERIALIZED_SIZE - (raw.len() - i);
        return Err(::bebop::DeserializeError::MoreDataExpected(missing));
    }

    Ok((i, Self {
    }))
}

Now it generates

fn _deserialize_chained(raw: &'raw [u8]) -> ::bebop::DeResult<(usize, Self)> {
    Ok((0, Self {}))
}