C++ Expected converted to idiomatic D

[Per Nordlöw]

On Tuesday, 28 August 2018 at 10:18:29 UTC, John Colvin wrote:
> I get the feeling from the talk that Andrei has some opinions 
> about how it should be done that aren't completely in line with 
> what has been proposed for the C++ standard.

Anyhow my implementation at

https://github.com/nordlow/phobos-next/blob/master/src/expected.d

should match his C++ code in the lecture, AFAICT.

My code doesn't depend on other modules an compiles very fast 
(~46 ms in my machine) (with DMD -debug -unittest).