message: Implement prevector::fill once
  Now that the implementation is identical, we can use a default value to
  distinguish them.
diff: @@ -196,11 +196,7 @@ private:
  T* item_ptr(difference_type pos) { return is_direct() ? direct_ptr(pos) : indirect_ptr(pos); }
  const T* item_ptr(difference_type pos) const { return is_direct() ? direct_ptr(pos) : indirect_ptr(pos); }
  - void fill(T* dst, ptrdiff_t count) {
  - std::fill_n(dst, count, T{});
  - }
  -
  - void fill(T* dst, ptrdiff_t count, const T& value) {
  + void fill(T* dst, ptrdiff_t count, const T& value = T{}) {
  std::fill_n(dst, count, value);
  }
  