std.data.json formal review

[Sönke Ludwig via Digitalmars-d]

> Anyway, I've just started to work on a generic variant of an enum based
> algebraic type that exploits as much static type information as
> possible. If that works out (compiler bugs?), it would be a great thing
> to have in Phobos, so maybe it's worth to delay the JSON module for that
> if necessary.
>

First proof of concept:
https://gist.github.com/s-ludwig/7a8a60150f510239f071#file-taggedalgebraic-d-L148

It probably still has issues with const/immutable and ref in some 
places, but the basics seem to work as expected.