1.0.45.25: better constant folding in arithmetic functions

1.0.45.25: better constant folding in arithmetic functions

 Patch by Heka Deep, lp#676414, edited to retain identities
 and added a random-tester.

 * Folds constants in expressions such as (+ 3 a 5 b 7 c).

 * Constants are collected by calling the `reduce-constants' function
   from the `source-transform-transitive' and
   `source-transform-intransitive' functions. Constants adding up to
   identities are retained so that SNaNs don't sneak past.