Support `Unalign` for unsized types?

Since `Unalign` is `repr(packed)`, `T` must be `Sized` (you can switch to `T: ?Sized + Copy`, but actually this is still `Sized` because `Copy: Clone` and `Clone: Sized`). We could lift this restriction by wrapping the inner `T` in a `ManuallyDrop`. However, we'd need to figure out what to do about dropping. We could implement `Drop` when `T: Sized` or when `T: Unaligned` (so dropping in-place is valid), but when `T: !Sized`, we'd either have to leak the type or else implement some crazy approximation of dynamic stack allocation in order to move the value into an aligned location in order to drop it in-place. Alternatively, perhaps we can prove that it's never possible to create an owned, unsized `Unalign`, and so the no-drop thing never matters in practice?

Background: If a type is `repr(packed)`, its fields may live at invalid offsets at runtime, which means that Rust must prevent those fields from being operated on in-place. If a field requires dropping, Rust generates drop glue for these fields by first moving them into the local stack frame with correct alignment. At this point, they can be safely dropped in-place. However, if the type is unsized, then there's no way for it to be moved into the local stack frame. Thus, Rust only allows unaligned fields which don't have drop glue. In the case of generic types, this requires wrapping in a `ManuallyDrop`.