Add `IncomingViewingKey::prepare` convenience method.

CHANGELOG.md

@@ -8,6 +8,7 @@ and this project adheres to Rust's notion of
 ## [Unreleased]
 
 ### Added
+- `orchard::keys::IncomingViewingKey::prepare`
 - `orchard::note::Rho`
 - `orchard::action::Action::rho`
 - `orchard::note_encryption::CompactAction::rho`

src/keys.rs

@@ -663,6 +663,11 @@ impl IncomingViewingKey {
     pub fn address(&self, d: Diversifier) -> Address {
         self.ivk.address(d)
     }
+
+    /// Returns the [`PreparedIncomingViewingKey`] for this [`IncomingViewingKey`].
+    pub fn prepare(&self) -> PreparedIncomingViewingKey {
+        PreparedIncomingViewingKey::new(self)
+    }
 }
 
 /// An Orchard incoming viewing key that has been precomputed for trial decryption.