Built-in Funtions

Based on these 2 ideas: https://www.outsystems.com/wisdomofthecrowds/IdeaComment_List.aspx?IdeaId=669
https://www.outsystems.com/wisdomofthecrowds/IdeaComment_List.aspx?IdeaId=396

I propose the following, that would satisfy everybody, I believe:

- The Index funtion could have 2 extra optional parameters:
       1 - indicating the direction of the search (left->right or right->left); (Default is left->right);
       2 - indicating the Index at wich the search is started at; (Default is 0 or end of string, depending on the direction parameter)
- In the Substr function, the "length" parameter could be optional. If empty than the function assumes "to the end of the string";

Although not as important as the above, I also propose 2 additional built-in math functions who already exist in C#:
- Floor(decimal)
- Ceiling(decimal)