Rework Rc for FCP of rust-lang#27718

* 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`.