bound attribute doesn't work with associated type bounds

Minimal example:

```rust
use arbitrary::Arbitrary;

pub trait Bar {
    type Value;
}

#[derive(Arbitrary)]
#[arbitrary(bound = "T::Value: Arbitrary")]
pub struct Foo<T: Bar>(T::Value);
```

Error output:

```
error: expected identifier
 --> src/lib.rs:8:21
  |
8 | #[arbitrary(bound = "T::Value: Arbitrary")]
  |                     ^^^^^^^^^^^^^^^^^^^^^
```