Try to add C++17 `template < class U = T > constexpr optional( U&& value )` constructor

In particular, this constructor would allow the following natural code:

```c++
auto add = [&options] (string o, optional<string> v = {})
{
  ...
};

add ("-usb");
add ("-device", "usb-kbd");
```

Note: propagate to `libbutl`.