add Kahan summation (numerically-accurate sum of floats) to List,Array,Enum

This change is inspired by
  http://re-factor.blogspot.fr/2013/10/kahan-summation.html
itself coming from
  http://adereth.github.io/blog/2013/10/10/add-it-up/

I discovered in the process that Kahan Summation was already
implemented in Batteries, for Enum. In this case, the existing
overhead of accessing the next element apparently made it an option
for the 'fsum' function already.
some benchmarks of Kahan summation
Merge pull request #459 from gasche/master

Kahan Summation in `Array`, `List` and `Enum`