Improving assert-printing in DMD

[Per Nordlöw via Digitalmars-d]

On Friday, 2 October 2015 at 14:54:08 UTC, Andrei Alexandrescu 
wrote:
> My first proposed lowering creates more copies than needed. My 
> second one is therefore probably better, without resorting to 
> compiler magic. -- Andrei

I update the spec. Could you take a look?

I'll update the operators, next.