std.manifestYamlDoc(object, quote_keys=false) is slow

When passing `quote_keys=false` to the method `std.manifestYamlDoc` it becomes really slow. Seems like the method checks if the key is safe to use without escaping. Uses the `bareSafe` method. This takes a lot of time.

Maybe the documentation should be updated to warn the users about it? Or the performance could be improved somehow. Or maybe let the users use force invalid keys without checking to lessen the performance hit?