Test derives on generic, unsized types

We currently don't have any tests for our derives on generic, unsized types such as:

```rust
#[derive(FromBytes, AsBytes, Unaligned)]
#[repr(transparent)]
struct Foo<T: ?Sized>(T);
```

```rust
#[derive(FromBytes, AsBytes, Unaligned)]
#[repr(packed)]
struct Bar<T: ?Sized>(T);
```