shared 'Header' behaviour

looking at the code, there's a lot of shared behaviour between the `UstarHeader` and `GnuHeader` types. There's also quite a lot of casting between the tops (which i guess is fine, since they're just representations of a vector of bytes anyway).

I wonder if the shared behaviour between these objects might be better represented as an `Enum` or as a trait, rather than being duplicated. Do you have any thoughts on that?