challenge #2: implement the varargs_reduce metafunction

[Andrei Alexandrescu (See Website for Email)]

My previous post defines max by availing itself of a metafunction called
varargs_reduce. Your challenge is to define it. varargs_reduce takes an
alias which it assumes is a function of two arguments, and it defines a
vararg function that applies that alias repeatedly, similarly to the
reduce primitive that is present in functional languages. Again, for
background on functional reduce, see e.g.:

http://www.joelonsoftware.com/items/2006/08/01.html

Define varargs_reduce to deduce the result type properly, and to 
generate code as efficient as possible.


Andrei