Apply cargo clippy suggestions

src/utils.rs

@@ -1,5 +1,7 @@
 #![allow(dead_code)]
 
+use std::fmt::Write;
+
 /// Returns a human readable String of a Duration
 ///
 /// Example: `3h 12m 53s`
@@ -7,7 +9,7 @@ pub fn format_duration(d: &std::time::Duration) -> String {
     let mut s = String::new();
     let mut append_unit = |value, unit| {
         if value > 0 {
-            s.push_str(&format!("{}{}", value, unit));
+            let _ = write!(s, "{}{}", value, unit);
         }
     };