## Motivation
The `tokio::sync::Semaphore` type provides a
[`Semaphore::available_permits` method][1] which returns the current
number of permits available on the semaphore.
`tokio_util::sync::PollSemaphore` [does not expose such a method][2]. It
is possible to use `PollSemaphore::into_inner` or
`PollSemaphore::clone_inner` to unwrap the inner semaphore, but this may
require cloning/dropping the semaphore's `Arc` which shouldn't be
necessary to access the permits.
## Solution
This commit adds `PollSemaphore::available_permits`. It also adds
`PollSemaphore::add_permits` and an `AsRef<Semaphore>` impl while
we're here.
[1]: https://docs.rs/tokio/1.4.0/tokio/sync/struct.Semaphore.html#method.available_permits
[2]: https://docs.rs/tokio-util/0.6.5/tokio_util/sync/struct.PollSemaphore.html#implementationsCloses#3682
* sync: move CancellationToken to tokio-util
The `CancellationToken` utility is only available with the
`tokio_unstable` flag. This was done as the API is not final, but it
adds friction for users.
This patch moves `CancellationToken` to tokio-util where it is generally
available. The tokio-util crate does not have any constraints on
breaking change releases.
* fix clippy
* clippy again
Taiki Endo