sync: add CancellationToken (tokio-rs#2263)

As a first step towards structured concurrency, this change adds a
CancellationToken for graceful cancellation of tasks.

The task can be awaited by an arbitrary amount of tasks due to the usage
of an intrusive list.

The token can be cloned. In addition to this child tokens can be derived.
When the parent token gets cancelled, all child tokens will also get
cancelled.

Author: Matthias247

tokio/Cargo.toml

@@ -123,6 +123,7 @@ optional = true
 [dev-dependencies]
 tokio-test = { version = "0.2.0", path = "../tokio-test" }
 futures = { version = "0.3.0", features = ["async-await"] }
+futures-test = "0.3.0"
 proptest = "0.9.4"
 tempfile = "3.1.0"