`CustomEncoder`/`CustomDecoder` improvements

[svix-daniel]

Ideally encoders and decoders should be improved such as to:

• Support &str in place of &String and &[T] in place of &Vec<T>
• Allow supporting trait objects?
• Allow "owned conversions"
• Use encoders/decoders over to_bytes_naive and from_bytes_naive when there is one to/from a byte array present.