der: extract a Decoder type in next breaking release #2533

der used to have a Decoder type, but it was refactored into SliceReader. Logic for handling nested productions winds up duplicated among the various *Reader impls in their Reader::read_nested methods, which are custom implemented per Reader type.

In ssh-encoding, which uses a similar arrangement of types, I introduced a Decoder type generic over a backend Reader which handles the central logic for consuming data from the readers, and all of the Decode methods now accept a &mut Decoder<impl Reader>: RustCrypto/SSH#618

In the next breaking release of der I think it'd be nice to do something similar. I added some partial shared logic in the Position type, but really it'd be nice to eliminate the duplication of logic between the reader types.