Make INSTR understand optional parameters for position and occurence

Currently the built-in function INSTR understands two parameters:

INSTR(string, substring)

We'll extend INSTR to understand two extra optional parameters:

INSTR(string, substring [,position [,occurence])

where:

• position is an nonzero integer indicating the character of string where to begin the search. If position is negative, then INSTR counts backward from the end of string and then searches backward from the resulting position.

• occurrence is a positive integer indicating which occurrence of string to search for.