challenge #2: implement the varargs_reduce metafunction

[janderson]

Andrei Alexandrescu (See Website For Email) wrote:

> 
> I think this ia a good point. Probably varargs_reduce must be linear to 
> make the most conservative assumptions. There is always the chance of 
> defining varargs_reduce_parallel. Thanks!
> 
> 
> Andrei

My thought exactly.  It would be nice to have a vargags_reduce and 
simply add _parallel to it for instant speed gains.  I also think that 
there will be other functions that could go the same way.  Perhaps 
_parallel (or whatever we decide to call it) should be standardized.