trace: Allow trace instrumentation to emit log records (#992)

## Motivation

`tokio-trace` currently offers a strategy for compatibility with the
`log` crate: its macros can be dropped in as a replacement for `log`'s
macros, and a subscriber can be used that translates trace events to log
records. However, this requires the application to be aware of
`tokio-trace` and manually set up this subscriber.

Many libraries currently emit `log` records, and would like to be able
to emit `tokio-trace` instrumentation instead. The `tokio` runtimes are
one such example. However, with the current log compatibility strategy,
replacing existing logging with trace instrumentation would break
`tokio`'s logs for any downstream user which is using only `log` and not
`tokio-trace`. It is desirable for libraries to have the option to emit
both `log` _and_ `tokio-trace` diagnostics from the same instrumentation
points.

## Solution

This branch adds a `log` feature flag to the `tokio-trace` crate, which
when set, causes `tokio-trace` instrumentation to emit log records as well
as `tokio-trace` instrumentation. 

## Notes

In order to allow spans to log their names when they are entered and 
exited even when the span is disabled, this branch adds an 
`&'static Metadata` to the `Span` type. This was previously stored in
the `Inner` type and was thus only present when the span was enabled.
This makes disabled spans one word longer, but enabled spans remain
the same size.

Fixes: #949

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

tokio-trace/examples/sloggish/sloggish_subscriber.rs

@@ -158,7 +158,7 @@ impl SloggishSubscriber {
             stderr: io::stderr(),
             stack: Mutex::new(vec![]),
             spans: Mutex::new(HashMap::new()),
-            ids: AtomicUsize::new(0),
+            ids: AtomicUsize::new(1),
         }
     }