Shared boxing and boxed self

### Summary of issue:


Reference counting and boxed `self`.

### Details:


#### Name

What naming convention are you going to use? E.g., `shared_ptr` and `atomic_ptr` map to...?

#### Self as any Boxed Type

```carbon
fn C.F[self: Atomic(Self)]() {}
```

### Any other information that you want to share?


_No response_