Extract Signed<T> to replace SignedStatement, SignedBitfield, etc

[coriolinus]

I think that it might make our life easier if we extracted signing behavior entirely with something like this:

struct Signed<T> {
    payload: T,
    validator_index: ValidatorIndex,
    signature: ValidatorSignature,
}

impl<T: Encode> Signed<T> {
    fn sign(payload: T, context: SigningContext, index: ValidatorIndex, key: PrivateKey) -> Signed<T> { ... }
    fn validate(&self, context: SigningContext, key: PublicKey) -> bool { ... }
}

Originally posted by @coriolinus in #1270

[rphmeier]

How would this handle the mutation of the signing payload used in Statement, where we replace the full AbridgedCandidateReceipt with its hash?

[coriolinus]

Via a slightly more complicated signature than is shown above:

polkadot/primitives/src/parachain.rs

Line 813 in dde56cc

pub struct Signed<Payload, RealPayload = Payload> {

polkadot/primitives/src/parachain.rs

Line 829 in dde56cc

impl<Payload: EncodeAs<RealPayload>, RealPayload: Encode> Signed<Payload, RealPayload> {

If you don't need to do any replacement, then you just use Signed<T>. For the replacement in Statement, we first impl EncodeAs<CompactStatement> for Statement, then use this type signature:

polkadot/node/primitives/src/lib.rs

Line 56 in dde56cc

pub type SignedStatement = Signed<Statement, CompactStatement>;

This keeps the nature of the encoding transparent to the user, so they never have to worry about it. As long as they're using the correct type, the right things happen.