compute the width of loading bar dynamically

src/lib.rs

@@ -40,11 +40,12 @@ where
     let mut times = vec![];
     for i in 0..b.nb_measurements {
         eprint!(
-            "bencher: {}, label: {} [{:>5}/{}]\r",
+            "bencher: {}, label: {} [{:>width$}/{}]\r",
             b.name,
             label,
             i + 1,
-            b.nb_measurements
+            b.nb_measurements,
+            width = format!("{}", b.nb_measurements).len()
         );
 
         times.push(f().as_nanos());