Missing `optional<T>::operator=(nullopt_t)`

Usually, this isn't noticable as the `nullopt` will implicitly convert to an `optional` during assignment. But for immovable types this fails to compile:

```c++
    beman::optional26::optional<immovable> o1(beman::optional26::in_place);
    o1 = beman::optional26::nullopt;
```