Convert BoxBytes to/from boxed slices

[zachs18]

As I mentioned in a comment on the BoxBytes PR, BoxBytes logically can be converted to/from boxed slices as well as boxed Sized types. This PR extends the existing box_bytes_of/(try_)from_box_bytes using sealed traits to allow them to take/return boxed slices as well as their current boxed Sized types (and Box<str> for box_bytes_of).

semver

The relaxation of the function bounds is not a breaking change.

The change from impl<T: NoUninit> From<Box<T>> for BoxBytes to impl<T: ?Sized + BoxBytesOf> From<Box<T>> for BoxBytes I believe is not a breaking change:

• Any type for which a downstream crate was previously allowed to implement From<Box<U>> for BoxBytes must have U as a local, non-NoUninit type, which as far as I can tell is still allowed under this new impl.

[Lokathor]

Released bytemuck-1.15.0

Please report any compatibility problems that somehow appear.