MSVC warning C4365: signed/unsigned mismatch in cxx.h iterator operator- overload

[wravery]

This is very similar to #1225, but I missed this one in cxx.h because it happened later in my own code once building the cxx crate dependency was unblocked:

template <typename T>
typename Slice<T>::iterator::difference_type
Slice<T>::iterator::operator-(const iterator &other) const noexcept {
  auto diff = std::distance(static_cast<char *>(other.pos),
                            static_cast<char *>(this->pos));
  return diff / this->stride;
}

It's reporting the same signed/unsigned warning (treated as an error in my case), but this time the issue is that this->stride is a size_t and diff and the typename Slice<T>::iterator::difference_type return type are both signed ptrdiff_t.

I should have a fix, but I want to make sure there aren't any more of these lurking in my code before I make another PR.