poor man's optional const&& constructor causes trouble in some cases

[eyalroz]

Trouble building with MSVC was reported in #664 , and the cause seems to be a gratuituous constructor of "poor man's optional":

poor_mans_optional &operator=(const T &&value) noexcept(::std::is_nothrow_move_assignable<T>::value)

I'm not quite sure why I introduced this in the first place, given that we already have a

poor_mans_optional &operator=(T &&value) noexcept(::std::is_nothrow_move_assignable<T>::value)

... so let's just remove this.