Get the offset value when converting to Vec

Calling `into_vec` will get a vec of the underlying data, but from what I can tell there's no way to access the "offset" at which the view into the underlying data started.  It would be great to have some way to get that information so even when using the raw bytes it'd be possible to know what bit offset the data started at; without it, `into_vec` seems like a "lossy" transformation.