pmars patch - fix maths evaluator

[Bjoern]

Something bit me and I decided to dust of my old copy of "The C
Programming Language" and try to fix a surprisingly severe pmars bug
in the maths evaluator: https://sourceforge.net/tracker/?func=detail&aid=621059&group_id=3452&atid=103452

With some luck I seem to have created a working patch, which I
submitted here: https://sourceforge.net/tracker/?func=detail&aid=2789845&group_id=3452&atid=303452

I am quite excited, as this is my first patch to an open source
project ever ;-)

I must admit I am not 100% sure that it works (the parser is quite
'magical'), but it succeeded in my tests. I would appreciate further
testing and/or comments.

I have used assertions in a test warrior (test_eval.red) to test all
kinds of maths expressions. Go wild ;-)

Björn