refactor: split the Header trait into three traits

[npmccallum]

The Header trait is now Named, Decodable and Encodable.

The goal of this change is to make it possible to implement Encodable
for borrowed types. Under the existing code, Header::decode() requires
that Self own its data. But it is a legitimate use to do
Header::encode() on borrowed data. By separating the traits, we can
implement Encodable on borrowed types and Encodable and Decodable
on owned types.

Signed-off-by: Nathaniel McCallum [email protected]