As part of this, we migrate to `secp256k1 0.27`. This version does not
bump `secp256k1-sys`, so remains compatible with the `libsecp256k1`
revision used in `zcashd`.
The `zcash_primitives::legacy::keys::AccountPrivKey` encoding also
changes to preserve the transparent extended key metadata. Previously
the type was documented as such, but only encoded the private key and
chain code; the new encoding now matches the documentation. As a side
effect, the unstable encoding of `zcash_keys::keys::UnifiedSpendingKey`
also changes.
Closeszcash/librustzcash#1407.
Closeszcash/librustzcash#1408.
As with our `cargo-vet` usage in `zcash/zcash`, these are binary crates
for interacting with Windows APIs, so both sides are maintained by
Microsoft and are not something we can audit ourselves.