sync: move CancellationToken to tokio-util (#2721)

* 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

1 files changed, 1 insertions, 0 deletions

diff --git a/tokio-util/src/loom.rs b/tokio-util/src/loom.rs
new file mode 100644
index 00000000..dd03feab
--- /dev/null
+++ b/tokio-util/src/loom.rs
@@ -0,0 +1 @@
+pub(crate) use std::sync;