Add methods for getting block nullifiers (#2465)

* Add methods for getting block nullifiers

These methods will be used in a future PR to check for double-spends.

* Add doc links

Co-authored-by: Janito Vaqueiro Ferreira Filho <janito.vff@gmail.com>

Co-authored-by: Janito Vaqueiro Ferreira Filho <janito.vff@gmail.com>

zebra-chain/src/block.rs

@@ -29,8 +29,11 @@ use serde::{Deserialize, Serialize};
 
 use crate::{
     fmt::DisplayToDebug,
+    orchard,
     parameters::{Network, NetworkUpgrade},
+    sapling,
     serialization::{TrustedPreallocate, MAX_PROTOCOL_MESSAGE_LEN},
+    sprout,
     transaction::Transaction,
     transparent,
 };
@@ -116,6 +119,30 @@ impl Block {
 
         Ok(())
     }
+
+    /// Access the [`sprout::Nullifier`]s from all transactions in this block.
+    pub fn sprout_nullifiers(&self) -> impl Iterator<Item = &sprout::Nullifier> {
+        self.transactions
+            .iter()
+            .map(|transaction| transaction.sprout_nullifiers())
+            .flatten()
+    }
+
+    /// Access the [`sapling::Nullifier`]s from all transactions in this block.
+    pub fn sapling_nullifiers(&self) -> impl Iterator<Item = &sapling::Nullifier> {
+        self.transactions
+            .iter()
+            .map(|transaction| transaction.sapling_nullifiers())
+            .flatten()
+    }
+
+    /// Access the [`orchard::Nullifier`]s from all transactions in this block.
+    pub fn orchard_nullifiers(&self) -> impl Iterator<Item = &orchard::Nullifier> {
+        self.transactions
+            .iter()
+            .map(|transaction| transaction.orchard_nullifiers())
+            .flatten()
+    }
 }
 
 impl<'a> From<&'a Block> for Hash {