Slightly simplify binary tree exercise (#2002)

Give students a little more context for the binary tree exercise by
giving them the wrapper methods on `BinaryTree` at the start and
explicitly asking them to implement the methods on `Subtree`. I think
this simplifies the exercise a bit and makes it a bit more focused for
students.

src/smart-pointers/exercise.rs

@@ -35,7 +35,6 @@ struct Subtree<T: Ord>(Option<Box<Node<T>>>);
 pub struct BinaryTree<T: Ord> {
     root: Subtree<T>,
 }
-// ANCHOR_END: types
 
 impl<T: Ord> BinaryTree<T> {
     fn new() -> Self {
@@ -54,6 +53,7 @@ impl<T: Ord> BinaryTree<T> {
         self.root.len()
     }
 }
+// ANCHOR_END: types
 
 impl<T: Ord> Subtree<T> {
     fn new() -> Self {