trace-core: API polish (#962)

This branch makes a handful of `tokio-trace-core` API improvements, mostly
around naming. In particular:

 * Rename `dispatcher::with` to `dispatcher::get_default`
 * Rename `Event::observe` to `Event::dispatch`
 * Make `field::ValidLen` trait private

Closes #948
Closes #960

Signed-off-by: Eliza Weisman <[email protected]>

tokio-trace/tests/support/subscriber.rs

@@ -185,7 +185,6 @@ where
     fn new_span(&self, span: &Attributes) -> Id {
         use span::Parent;
         let meta = span.metadata();
-        let values = span.values();
         let id = self.ids.fetch_add(1, Ordering::SeqCst);
         let id = Id::from_u64(id as u64);
         println!(