Truncate secret hash using precision

Currently we are attempting to truncate the hash created using
`bitcoin_hashes` by using the "width" formatting parameter instead of
the "precision" parameter. `hex-conservative` truncates with the
"precision" parameter as is expected since a hash is not an integral
type.

Use the formatting string `"{:.16}"` which is the "precision"
formatting parameter.

src/secret.rs

@@ -46,7 +46,7 @@ macro_rules! impl_display_secret {
                 engine.input(&self.secret_bytes());
                 let hash = sha256::Hash::from_engine(engine);
 
-                f.debug_tuple(stringify!($thing)).field(&format_args!("#{:016x}", hash)).finish()
+                f.debug_tuple(stringify!($thing)).field(&format_args!("#{:.16}", hash)).finish()
             }
         }