Rework Arc for FCP of rust-lang#27718

* 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`.