[Wish]_Built-in text functions

A few suggestions for new built-in text functions:

- Index() with StartAt parameter, as in C# IndexOf (I frequently use an extension for this one);

- a better way of getting one character from a string (at location x) than Substr() (it seems cumbersome for this);

- SubstrRight(t, n) - returns n chars from t, counting from the right.

Paulo Ramos