allow slice and mut slice views of storage

cc @vadixidav

It would be convenient to view the underlying storage as a raw slice (and mut slice). I see `.as_slice()` and `.as_mut_slice()`, but these require `ContiguousStorage` and `ContiguousStorageMut` bounds. It would be useful to get slices to the underlying data also for the cases in which the data is not `ContiguousStorage`. This would be useful to remove unsafe in the [nshare](https://github.com/rust-cv/nshare) crate, which currently uses `.as_ptr()`.