New syntax for string mixins

[Graham St Jack]

> I've attached a part of how concurrency.d could look like translated 
> to my suggested syntax. It probably contains a lot of errors because 
> did a quick translation and I had some trouble understanding the mixins.
>
Yes, even I couldn't understand them even a week later. Maintenance is a 
real problem.

My initial reaction is that the proposed syntax helps a bit, but isn't 
really a game-changer. I will let you know if I change my mind on closer 
inspection.

-- 
Graham St Jack