update for ptr provenance

[RalfJung]

Adds a new data structurue, mono_hash_map, which uses RefCell to be able to insert new entries even when we just have a shared ref. However, it is important that we can also return shared refs into the map without holding the RefCell open. This is achieved by boxing the values stored in the map, so their addresses remain stable even when the map's table gets reallocated. This is all implemented in mono_hash_map.rs.

Waiting for rust-lang/rust#54461