Use precision when formatting core::hash::Hasher

It seems the `core::hash::Hasher` supports precision as well as width
formatting but defaults to displaying as an integer.

To be uniform with our usage of `hashes::sha256::Hash` type (and
unsurprising to the reader) use the precision formatting parameter.

Note this does not change the output, the unit test is not touched.

src/secret.rs

@@ -28,7 +28,7 @@ macro_rules! impl_display_secret {
                 hasher.write(&self.secret_bytes());
                 let hash = hasher.finish();
 
-                f.debug_tuple(stringify!($thing)).field(&format_args!("#{:016x}", hash)).finish()
+                f.debug_tuple(stringify!($thing)).field(&format_args!("#{:.16x}", hash)).finish()
             }
         }