message: Don't allocate in `SourceId::is_default_registry`
  This gets called quite a lot and doesn't need to allocate in the first
  place!
diff: @@ -337,7 +337,7 @@ impl SourceId {
  Kind::Registry => {}
  _ => return false,
  }
  - self.inner.url.to_string() == CRATES_IO_INDEX
  + self.inner.url.as_str() == CRATES_IO_INDEX
  }
  /// Hashes `self`.
  