As of zcash/librustzcash#633, `SaplingDomain::IncomingViewingKey` now
allocates memory internally, and this memory persists as long as the
`BatchRunner` is alive. Now that we have decoupled the measurement of
heap usage for batch tasks from their internals, we can add bounds to
all of the generic parameters of `Batch` to enable correctly measuring
their actual heap usage.
Ported from zcash/librustzcash@913aa0a988.
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.