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/examples/counters.rs

@@ -2,7 +2,7 @@
 extern crate tokio_trace;
 
 use tokio_trace::{
-    field::{self, Field, Visit},
+    field::{Field, Visit},
     span,
     subscriber::{self, Subscriber},
     Event, Id, Metadata,