Make Arc::make_unique check weak refs; add make_unique to Rc

This patch makes `Arc::make_unique` examine the number of weak
references as well as strong references, which is required for safety.

It also adds a `make_unique` method to the `Rc` type for consistency.

Closes #14521.
auto merge of #14522 : aturon/rust/make_unique, r=alexcrichton,alexcrichton,me

This patch makes `Arc::make_unique` examine the number of weak
references as well as strong references, which is required for safety.

It also adds a `make_unique` method to the `Rc` type for consistency.

Closes #14521.