refac(rust): use hashmap entry for in-place manipulation

rust/tests/2_counting_duplicates.rs

@@ -4,7 +4,7 @@ fn count_duplicates(text: &str) -> u32 {
     text.to_lowercase()
         .chars()
         .fold(HashMap::<char, u32>::new(), |mut hm, c| {
-            hm.insert(c, hm.get(&c).copied().unwrap_or_default() + 1);
+            (*hm.entry(c).or_default()) += 1;
             hm
         })
         .values()