Support transaction batch signatures

[imalsogreg]

Add a signature scheme for batch signatures.

There will be a KIP for this soon, and we will link to it from this PR.

Batch signatures are a type of signature that can be assigned to multiple transactions at once. One part of the signature (the root) is shared across all transactions. Another part (the proof) is specific to each transaction in the batch.

Batch signatures are implemented with Merkle trees. The root of the signature is a Merkle root of all the transactions in the batch, hashed with Blake2b_256. Each transactions proof is a Merkle proof of that transaction's inclusion in the batch.

PR checklist:

• Test coverage for the proposed changes
• PR description contains example output from repl interaction or a snippet from unit test output
• (If Relevant) Documentation has been (manually) updated at https://docs.kadena.io/pact

Additionally, please justify why you should or should not do the following:

• Benchmark regressions
• Confirm replay/back compat (Ignore until core release)
• (For Kadena engineers) Run integration-tests against a Chainweb built with this version of Pact (Ignore until core release)

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