Remove Mref

[danakj]

I think it's a cool idea but the mref() annotation should be done through a static analysis check instead of in the type system.

By doing it in the type system, it forces function/method overloads when receiving const T& or T& in a template where the distinction matters. Overloading is against the principles. For example in Option:

  constexpr T& insert(T t) & noexcept
    requires(sus::mem::Move<T> &&
             !(std::is_reference_v<T> &&
               !std::is_const_v<std::remove_reference_t<T>>))
  {
    t_.set_some(move_to_storage(t));
    return t_.val_;
  }

  constexpr T& insert(Mref<std::remove_reference_t<T>> t) & noexcept
    requires(std::is_reference_v<T> &&
             !std::is_const_v<std::remove_reference_t<T>>)
  {
    t_.set_some(move_to_storage(t));
    return t_.val_;
  }

Keep mref()s

In anticipation of a static check, keep the mref() annotations that we have, but make it a function

template <class T>
decltype(auto) mref(T&& t) { return static_cast<T&&>(t); }