message: Shrink overallocated capacity if a number gets a lot smaller.
  If a number gets below 1/4 of capacity, shrink_to_fit. Since Vec grows by 2, this amortizes well
  for any sequence of sizes.
diff: @@ -2617,6 +2617,9 @@ impl BigUint {
  while let Some(&0) = self.data.last() {
  self.data.pop();
  }
  + if self.data.len() < self.data.capacity() / 4 {
  + self.data.shrink_to_fit();
  + }
  }
  /// Returns a normalized `BigUint`.
  