digest: implement Zeroize for the buffering newtypes

Author: baloo

digest/src/buffer_macros/fixed.rs

@@ -39,6 +39,21 @@ macro_rules! buffer_fixed {
             const OID: $crate::const_oid::ObjectIdentifier =
                 $crate::const_oid::ObjectIdentifier::new_unwrap($oid);
         }
+
+        impl$(< $( $lt $( : $clt $(+ $dlt )* )? ),+ >)? Drop for $name$(< $( $lt ),+ >)? {
+            #[inline]
+            fn drop(&mut self) {
+                #[cfg(feature = "zeroize")]
+                {
+                    use $crate::zeroize::Zeroize;
+                    self.0.zeroize();
+                }
+            }
+        }
+
+        #[cfg(feature = "zeroize")]
+        impl$(< $( $lt $( : $clt $(+ $dlt )* )? ),+ >)? $crate::zeroize::ZeroizeOnDrop for $name$(< $( $lt ),+ >)? {}
+
     };
 
     // Terminates `impl_inner` sequences.