Implement Debug when no std feature

The `Debug` implementation for secrets is feature gated on `std`, this
means we cannot build cleanly with `--no-default-features`.

When the `std` feature is not enabled implement a dummy version of
`Debug` that just displays

   <secret requires std feature to display>

src/secret.rs

@@ -44,6 +44,13 @@ macro_rules! impl_display_secret {
                     .finish()
             }
         }
+
+        #[cfg(not(feature = "std"))]
+        impl ::core::fmt::Debug for $thing {
+            fn fmt(&self, f: &mut ::core::fmt::Formatter) -> ::core::fmt::Result {
+                write!(f, "<secret requires std feature to display>")
+            }
+        }
      }
 }