A unique string key for TypeId #21116

I want to implement a mapping of Box<T: 'static> to GType. That dynamic type system requires process-unique type names. So I'd like to have a process-unique key, preferably a string, available from TypeId or a similar API.

Now, it's likely that TyDesc names are sufficiently unique, and hash collisions on TypeId break the world as Rust knows it (#17179) so the u64 hash value can be used as practically unique, but: 1) uniqueness constraints on any of those are not documented; 2) access to TyDesc is unsafe.