impl<T> Ranged for &T where T: Ranged (#16)

In the example below, `arg` is `&Expr`, so `&Ranged`, but `entries()`
want a `T: Ranged`. This adds the missing bridge impl.

```rust
        let all_args = format_with(|f| {
            f.join_comma_separated()
                .entries(
                    // We have the parentheses from the call so the arguments never need any
                    args.iter()
                        .map(|arg| (arg, arg.format().with_options(Parenthesize::Never))),
                )
                .nodes(keywords.iter())
                .finish()
        });
```

ast/src/ranged.rs

@@ -14,4 +14,13 @@ pub trait Ranged {
     }
 }
 
+impl<T> Ranged for &T
+where
+    T: Ranged,
+{
+    fn range(&self) -> TextRange {
+        T::range(self)
+    }
+}
+
 include!("gen/ranged.rs");