Make the inner type wrapped in `Unalign` public

This is not exactly a customer request since I'm not using this crate currently, but I was looking on the docs and I wondered why won't you make the inner field of [`Unalign`](https://docs.rs/zerocopy/latest/zerocopy/struct.Unalign.html) public?

Usually when prefer to make newtype's field private, but in this case making it public has a big advantage: we cannot make a reference to it and so we cannot expose it in a non-copying way, but if it is public the compiler allows reads/writes to fields of it as long as we never take a reference and only read/write directly. This can make working with this type more comfortable.