Implement `AsRef` for casting to superclasses

If you have a reference `b_ref: &B`, and `B` is a subclass of `A`, it would sometimes be useful to have an `&A` in order to pass into APIs which expect a `&A`. Experiments suggest that this is ergonomic using `AsRef`, at least for constant references (not so much for pinned mutable references).

Start by reviewing the issue's experiments with Rust AsRef and the repository's Rust/C++ interop implementation. Determine how constant references from a subclass should convert to superclass references, including the stated limitations around pinned mutable references; done means the supported conversion is implemented and covered by tests.

Written by the indexing model from the issue text.