Currently supports Zcash blocks, block headers, and transactions. Some
consensus rules are also checked, and a JSON context object can be
optionally passed to provide any necessary details for extra contextual
consensus checks.
The primary purpose of this commit is an exercise in using `cargo vet`
for tracking audits of our Rust dependency updates. `cargo update` was
run, and then a simple-to-audit subset of the dependency updates were
audited and committed.