Add DiversifiableFullViewingKey::to_internal_fvk.

This is needed in order to permit spending of internal notes after the
previous change to `Builder::add_spend`.

CHANGELOG.md

@@ -16,6 +16,7 @@ and this library adheres to Rust's notion of
   - `SpendAuthorizingKey::to_bytes`
   - `SpendValidatingKey::to_bytes`
 - `sapling_crypto::value::ValueSum::to_raw`
+- `sapling_crypto::zip32::DiversifiableFullViewingKey::to_internal_fvk`
 
 ### Fixed
 - `sapling_crypto::prover::OutputProver::prepare_circuit` now takes `esk` as an

src/pczt.rs

@@ -25,7 +25,7 @@ mod io_finalizer;
 pub use io_finalizer::IoFinalizerError;
 
 mod updater;
-pub use updater::{SpendUpdater, OutputUpdater, Updater, UpdaterError};
+pub use updater::{OutputUpdater, SpendUpdater, Updater, UpdaterError};
 
 mod prover;
 pub use prover::ProverError;

src/zip32.rs

@@ -705,6 +705,11 @@ impl DiversifiableFullViewingKey {
         &self.fvk
     }
 
+    /// Returns the internal [`FullViewingKey`] component of this diversifiable full viewing key.
+    pub fn to_internal_fvk(&self) -> FullViewingKey {
+        self.derive_internal().fvk
+    }
+
     /// Derives a nullifier-deriving key for the provided scope.
     ///
     /// This API is provided so that nullifiers for change notes can be correctly computed.