Use derived equality and ordering (which delegate to constant-time
versions) for note::nullifier::Nullifier and tree::MerkleHashOrchard
so that these types can be used as map keys in wallets.
We move Clippy lints into separate workflows that only run either
directly on branches, or on trial-merges of PRs, to deduplicate the
lint annotations. We will still see stable lints duplicated in nightly,
but that's fine.