Method to_utf32

Fleece/Support/diff_match_patch.hh:2536–2540

struct diff_match_patch_utf32_direct {
  typedef utf32_type utf32_t;
  template <class iterator> static iterator to_utf32(iterator i, iterator /*end*/, utf32_t& u)
  {
    u = *i++;
    return i;
  }
  template <class iterator> static iterator from_utf32(utf32_t u, iterator o)
  {
    *o++ = static_cast<char_t>(u);