Switch to the HashMap's Entry API

[jalil-salame]

Improvement:

• When doing if get_mut(..) else .insert it doesn't rehash the key
• It is arguably more readable

Problems:

• When filling up the df HashMap, it clones the string unnecessarily:

  Old code:

  if let Some(freq) = self.df.get_mut(t) {
      *freq += 1;
  } else {
      // `t` is only cloned when the key is not present
      self.df.insert(t.clone(), 1);
  }

  New code:

  // Always clones `t`
  self.df.entry(t.clone())
      .and_modify(|freq| *freq += 1)
      .or_insert(1);

• Uses closures so performance in debug builds might degrade