Add signatures to data structures; revise validation model

[tiller-tolbus]

txn and block both need to have signatures attached as we will not be verifying data by virtue of the network alone, per #2.

Within the %cast handler of the agent, we will then need to add some signature validation logic in place of the remote scry attempt. We might also just want to remove the remote scry at that specific location, as adding an entire new network message here reduces performance and reliability, and is not useful for validation.

[tiller-tolbus]

Signatures are now supported and in use to sign transactions and blocks.

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