substring: Define negative index for substring function

In ODATA-781 we defined that a negative index in substring is explicitly undefined, since we did not define a behavior and there were at least two valid interpretations, and that interoperable clients should not use negative indexes. In 4.01, we agreed that negative indexes should mean count from the end. in particular, substring(string,-n,m) is equivalent to substring(string,length(string)-n,m)