Implement accessors for the Tracer

This makes the `Tracer` wrapper for a phy device close to a drop-in
replacement for the encapsulated device.

Closes: #289
Approved by: whitequark

src/phy/tracer.rs

@@ -19,6 +19,21 @@ impl<D: for<'a> Device<'a>, P: PrettyPrint> Tracer<D, P> {
         Tracer { inner, writer }
     }
 
+    /// Get a reference to the underlying device.
+    ///
+    /// Even if the device offers reading through a standard reference, it is inadvisable to
+    /// directly read from the device as doing so will circumvent the tracing.
+    pub fn get_ref(&self) -> &D {
+        &self.inner
+    }
+
+    /// Get a mutable reference to the underlying device.
+    ///
+    /// It is inadvisable to directly read from the device as doing so will circumvent the tracing.
+    pub fn get_mut(&mut self) -> &mut D {
+        &mut self.inner
+    }
+
     /// Return the underlying device, consuming the tracer.
     pub fn into_inner(self) -> D {
         self.inner