generalize SignedTxData to allow multiple signatures

[tzemanovic]

This is to allow transactions with multiple signatures (e.g. multi-sig VP or multiple transfers from different sources bundled in one tx).

A possible approach would be to replace the sig field with e.g.

pub struct SignedTxData {
    ...
    /// The signatures are produced on the unsigned tx (tx data concatenated
    /// with the tx code and the timestamp).
    pub sigs: BTreeMap<PublicKeyHash, common::Signature>,
}

So a VP looking for a signature can look it up from the sigs using the hash of its PK.

[cwgoes]

We should figure out a good generalised way to encode arbitrary VP-required data, provided by intents, in transactions - for example, we could have transactions include a VP-address-indexed map of byte arrays (which can then be encoded in a way understood by the validity predicate, e.g. as a set of signatures).

[cwgoes]

@grarco what is the status of this w.r.t. the multisignature VP?

[grarco]

We started addressing this with the specs of #680. Should we close this in favor of #81 since they look very similar?

[grarco]

Close in favor of #81