Convenience functions for sum and product across a Collection type

[scanon (Steve Canon)]

@Nevin

I think sum() belongs in either the standard library or perhaps Numerics, rather than Algorithms.

I would like to add the full set of IEEE 754 reduction operations in Numerics: sum, dot, sumSquare, sumAbs, scaledProd, scaledProdSum, scaledProdDiff (IEEE names used here; we would be somewhat more verbose, I expect). sum is as good a place to start as any.