Improve `#[derive(IntoBytes)]` padding analysis on DSTs

Many dynamically sized types can be `derive(FromBytes)` but not `derive(IntoBytes)` because of shortcomings in our padding analysis. For example:

```rust
#[derive(KnownLayout, FromBytes, IntoBytes)]
#[repr(C)]
struct Example {
header: Header,
body: [Body],
}
```

Presently, a user must either mark `Example` as `repr(C, packed)`, or they must model `Example` instead as a composite struct containing `Ref`s to the `Header` and `[Body]`.

Can we make our analysis more sophisticated in this case?