Fix pattern matching ref/deref

Clippy emits:

 warning: deref on an immutable reference

Remove the explicit reference and use `ref` when pattern matching.

Author: tcharding

src/descriptor/tr.rs

@@ -354,8 +354,8 @@ where
     fn next(&mut self) -> Option<Self::Item> {
         while !self.stack.is_empty() {
             let (depth, last) = self.stack.pop().expect("Size checked above");
-            match &*last {
-                TapTree::Tree(l, r) => {
+            match *last {
+                TapTree::Tree(ref l, ref r) => {
                     self.stack.push((depth + 1, r));
                     self.stack.push((depth + 1, l));
                 }