book-store: explicitly implement PartialEq

clippy:
error: you are implementing `Hash` explicitly but have derived `PartialEq`
note: `#[deny(clippy::derive_hash_xor_eq)]` on by default
help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#derive_hash_xor_eq

Author: petertseng

exercises/book-store/example.rs

@@ -10,7 +10,7 @@ type GroupedBasket = Vec<Group>;
 type Price = u32;
 const BOOK_PRICE: Price = 800;
 
-#[derive(Debug, Clone, PartialEq, Eq)]
+#[derive(Debug, Clone)]
 struct Group(RefCell<BTreeSet<Book>>);
 
 impl Group {
@@ -65,6 +65,14 @@ impl PartialOrd for Group {
     }
 }
 
+impl PartialEq for Group {
+    fn eq(&self, other: &Group) -> bool {
+        self.0.borrow().eq(&other.0.borrow())
+    }
+}
+
+impl Eq for Group {}
+
 impl Hash for Group {
     fn hash<H: Hasher>(&self, hasher: &mut H) {
         self.0.borrow().hash(hasher);