subscriber: Forward current_span impl for Layered (#350)

This overrides the default `Subscriber::current_span` impl
with one that forwards the inner subscribers implementation.
This fixes bugs where the current_span was not fetchable when
using the `FmtSubscriber`.

Signed-off-by: Lucio Franco <luciofranco14@gmail.com>

tracing-subscriber/src/layer.rs

@@ -501,6 +501,11 @@ where
         }
     }
 
+    #[inline]
+    fn current_span(&self) -> span::Current {
+        self.inner.current_span()
+    }
+
     #[doc(hidden)]
     unsafe fn downcast_raw(&self, id: TypeId) -> Option<*const ()> {
         self.layer