Apply some clippy fixes.

build.rs

@@ -14,7 +14,7 @@ use walkdir::WalkDir;
 fn main() {
     let out_dir = env::var("OUT_DIR").unwrap();
     let destination = Path::new(&out_dir).join("tests.rs");
-    let mut test_file = File::create(&destination).unwrap();
+    let mut test_file = File::create(destination).unwrap();
 
     // Re-generate the tests if any org-mode files change
     println!("cargo:rerun-if-changed=org_mode_samples");
@@ -51,7 +51,7 @@ fn write_test(test_file: &mut File, test: &walkdir::DirEntry) {
         .to_lowercase()
         .strip_suffix(".org")
         .expect("Should have .org extension")
-        .replace("/", "_");
+        .replace('/', "_");
 
     write!(
         test_file,