Method constexpr_write_ptr

cpp/simdutf.h:1428–1428

 */
template <typename SrcType, typename TargetType> struct constexpr_write_ptr {
  constexpr explicit constexpr_write_ptr(TargetType *raw) : p(raw) {}

  constexpr constexpr_write_ptr_proxy<SrcType, TargetType> operator*() const {
    return constexpr_write_ptr_proxy<SrcType, TargetType>{p};