Rust Serde Deserialize

[Rajiv M Ranganath]

Hi,

I am new to Avro. When I have an Arvo schema of the form,

```
record Abcd {
    union { null, bytes } efgh = null;
}
```

What would be the corresponding Rust struct?

I tried

```
#[derive(Debug, Deserialize, PartialEq)]
struct Abcd {
    efgh: Option<Vec<u8>>,
}
```

But for some reason, `apache_avro::from_value` is giving an
`Err(DeserializeValue("not an array"))` error.

Best,
Rajiv