Be more consistent about tests vs. main (#2644)

The content slides all use `fn main`, with the exception of the testing
segment. But with this change, where it makes sense exercises use tests
instead, and not both tests and `fn main`.

A small change in `book.js` supports running tests when a code sample
does not have `fn main` but does have `#[test]`, so these work
naturally.

Fixes #1581.

src/smart-pointers/Cargo.toml

@@ -4,6 +4,6 @@ version = "0.1.0"
 edition = "2021"
 publish = false
 
-[[bin]]
-name = "binary-tree"
+[lib]
+name = "binary_tree"
 path = "exercise.rs"

src/smart-pointers/exercise.md

@@ -14,7 +14,7 @@ Implement the following types, so that the given tests pass.
 Extra Credit: implement an iterator over a binary tree that returns the values
 in order.
 
-```rust,editable,ignore
+```rust,compile_fail,editable
 {{#include exercise.rs:types}}
 
 // Implement `new`, `insert`, `len`, and `has` for `Subtree`.

src/smart-pointers/exercise.rs

@@ -11,6 +11,7 @@
 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 // See the License for the specific language governing permissions and
 // limitations under the License.
+#![allow(dead_code)]
 
 // ANCHOR: solution
 use std::cmp::Ordering;
@@ -96,14 +97,6 @@ impl<T: Ord> Node<T> {
     }
 }
 
-fn main() {
-    let mut tree = BinaryTree::new();
-    tree.insert("foo");
-    assert_eq!(tree.len(), 1);
-    tree.insert("bar");
-    assert!(tree.has(&"foo"));
-}
-
 // ANCHOR: tests
 #[cfg(test)]
 mod tests {