Expose task ID in task dumps

[jswrenn]

Is your feature request related to a problem? Please describe.
There is presently no mechanism for following the state of a task between successive dumps.

Describe the solution you'd like
Task dumps should include task IDs. It is probably permissible for these IDs to be occasionally recycled.

[jswrenn]

@Darksonn I see that Tokio already has something called task_id in Core:

tokio/tokio/src/runtime/task/core.rs

Lines 137 to 146 in 131e7b4

	pub(super) struct Core<T: Future, S> {
	/// Scheduler used to drive this future.
	pub(super) scheduler: S,
	/// The task's ID, used for populating `JoinError`s.
	pub(super) task_id: Id,
	/// Either the future or the output.
	pub(super) stage: CoreStage<T>,
	}

...but I can't immediately follow how to go from a &Task to task_id.

[Darksonn]

See:

tokio/tokio/src/runtime/task/join.rs

Lines 297 to 311 in 131e7b4

	/// Returns a [task ID] that uniquely identifies this task relative to other
	/// currently spawned tasks.
	///
	/// **Note**: This is an [unstable API][unstable]. The public API of this type
	/// may break in 1.x releases. See [the documentation on unstable
	/// features][unstable] for details.
	///
	/// [task ID]: crate::task::Id
	/// [unstable]: crate#unstable-features
	#[cfg(tokio_unstable)]
	#[cfg_attr(docsrs, doc(cfg(tokio_unstable)))]
	pub fn id(&self) -> super::Id {
	// Safety: The header pointer is valid.
	unsafe { Header::get_id(self.raw.header_ptr()) }
	}