Expose `Bytes` instance from `Unreal4File`.

[Dav1dde]

Relay parses Unreal Files and needs ownership of the bytes, which currently requires cloning the data, since symbolic only exposes access to a slice.

Reason why we might not want to do this:

• It adds Bytes to the public interface and leaks the implementation detail, which will tie us to it until the next breaking change

[linear]

SYMBOL-20