subscriber: Implement FollowsFrom

[davidbarsky]

(This is followup from #420)

The Registry and fmt::Layer should record all the spans that a given span succeeds. This data will need to be added to DataInner:

tracing/tracing-subscriber/src/registry/sharded.rs

Lines 67 to 72 in 773c440

	struct DataInner {
	metadata: &'static Metadata<'static>,
	parent: Option<Id>,
	ref_count: AtomicUsize,
	pub(crate) extensions: RwLock<ExtensionsInner>,
	}

and exposed via the SpanData trait:

tracing/tracing-subscriber/src/registry/mod.rs

Lines 162 to 183 in 773c440

	pub trait SpanData<'a> {
	/// Returns this span's ID.
	fn id(&self) -> Id;
	/// Returns a reference to the span's `Metadata`.
	fn metadata(&self) -> &'static Metadata<'static>;
	/// Returns a reference to the ID
	fn parent(&self) -> Option<&Id>;
	/// Returns a reference to this span's `Extensions`.
	///
	/// The extensions may be used by `Layer`s to store additional data
	/// describing the span.
	fn extensions(&self) -> Extensions<'_>;
	/// Returns a mutable reference to this span's `Extensions`.
	///
	/// The extensions may be used by `Layer`s to store additional data
	/// describing the span.
	fn extensions_mut(&self) -> ExtensionsMut<'_>;
	}

Note that FollowsFrom is a singly-linked graph, so that FollowsFrom relationship will need to be stored as a Vec<Id> on DataInner.