Implement Borrow<T> for Handle<T>

This makes `Handle` more useful in some generic code, and I'm about to
make use of that.

crates/fj-core/src/storage/handle.rs

@@ -1,4 +1,6 @@
-use std::{any::type_name, cmp::Ordering, fmt, hash::Hash, ops::Deref};
+use std::{
+    any::type_name, borrow::Borrow, cmp::Ordering, fmt, hash::Hash, ops::Deref,
+};
 
 use super::{blocks::Index, store::StoreInner};
 
@@ -82,6 +84,12 @@ impl<T> Deref for Handle<T> {
     }
 }
 
+impl<T> Borrow<T> for Handle<T> {
+    fn borrow(&self) -> &T {
+        self.deref()
+    }
+}
+
 impl<T> Clone for Handle<T> {
     fn clone(&self) -> Self {
         Self {