derive on generic types

It doesn't seem like it's possible to do
```rust
#[derive(Debug, XmlRead, XmlWrite)]
#[xml(tag = "Packet")]
struct Packet<T>
where
    T: XmlRead + XmlWrite,
{
    #[xml(flatten_text = "Command")]
    command: Command,
    #[xml(child = "DatabaseManager")]
    database_manager: T,
}
```
Would it be possible to move the trait bounds into the `impl`, the way `serde` does it?