Add prevHash field to CompactBlock

This enables basic verification of chain validity when CompactBlocks are
received without the full header.

zcash_primitives/src/block.rs

@@ -22,6 +22,20 @@ impl fmt::Display for BlockHash {
     }
 }
 
+impl BlockHash {
+    /// Constructs a [`BlockHash`] from the given slice.
+    ///
+    /// # Panics
+    ///
+    /// This function will panic if the slice is not exactly 32 bytes.
+    pub fn from_slice(bytes: &[u8]) -> Self {
+        assert_eq!(bytes.len(), 32);
+        let mut hash = [0; 32];
+        hash.copy_from_slice(&bytes);
+        BlockHash(hash)
+    }
+}
+
 /// A Zcash block header.
 pub struct BlockHeader {
     hash: BlockHash,