Remember that this was an API, exposing the `&[Attributes]` to external consumers. I may not want to hold pointers to the insides, but a consumer of the API mig

Remember that this was an API, exposing the `&[Attributes]` to external consumers. I may not want to hold pointers to the insides, but a consumer of the API might. Also, iterator invalidation. Also, runtime overhead of shared_ptr. (I added a footnote to the post to explain the issues with shared pointers)