Auto merge of #27837 - Gankro:weaknique, r=aturon

This prepares both for the FCP of #27718

Arc:

* Add previously omitted function `Arc::try_unwrap(Self) -> Result<T, Self>`
* Move `arc.downgrade()` to `Arc::downgrade(&Self)` per conventions.
* Deprecate `Arc::weak_count` and `Arc::strong_count` for raciness. It is almost
  impossible to correctly act on these results without a CAS loop on the actual
  fields.
* Rename `Arc::make_unique` to `Arc::make_mut` to avoid uniqueness terminology 
  and to clarify relation to `Arc::get_mut`.
 

Rc:
* Add `Rc::would_unwrap(&Self) -> bool` to introspect whether try_unwrap would succeed, 
  because it's destructive (unlike get_mut).
* Move `rc.downgrade()` to `Rc::downgrade(&Self)` per conventions.
* Deprecate `Rc::weak_count` and `Rc::strong_count` for questionable utility.
* Deprecate `Rc::is_unique` for questionable semantics (there are two kinds of 
  uniqueness with Weak pointers in play).
* Rename `rc.make_unique()` to `Rc::make_mut(&mut Self)` per conventions, to
  avoid uniqueness terminology, and to clarify the relation to `Rc::get_mut`.

Notable omission:

* Arc::would_unwrap is not added due to the fact that it's racy, and therefore doesn't 
  provide much actionable information.

(note: rc_would_unwrap is not proposed for FCP as it is truly experimental)


r? @aturon (careful attention needs to be taken for the new Arc::try_unwrap, but intuitively it should "just work" by virtue of being semantically equivalent to Drop).

Author: bors