C++ Expected converted to idiomatic D

[Per Nordlöw]

In

https://www.youtube.com/watch?v=nVzgkepAg5Y

Andrei describes his proposal for STL `Expected` planned to be 
included in C++20.

Have anybody converted the C++ proposal to idiomatic D, yet?

Hopefully without the pointer-legacy which unfortunately was 
allowed into `std:optional`.

Andrei claims we should it as return types for non-throwing 
variants of parse() and to() in the works at

https://github.com/dlang/phobos/pull/6665