Make doughnut a versioned enum

[jordy25519]

/// A versioned doughnut wrapper.
/// It proxies to the real,inner doughnut type
pub enum Doughnut {
   /// A v0 doughnut
    V0(v0::DoughnutV0),
}

It should implement decode, encode, DoughnutAPI and DoughnutVerify and simply match over the variants to proxy to the inner type.