Use #[derive(RlpEncodableWrapper, RlpDecodableWrapper)]

```rust
struct StructWithSingleField {
field: T
}

impl Encodable for StructWithSingleField {
fn rlp_append(&self, s: &mut RlpStream) {
self.field.rlp_append(s);
}
}
```
can be simplified to
```rust
#[derive(RlpEncodableWrapper)]
struct StructWithSingleField {
field: T
}
```

It can be applied to `struct StructWithSingleTuple(T)`, and `RlpDecodable` too.