Improved Intersect Functionality

[marcinas]

The intersect functionality for pdp expressions as it is currently implemented consists of 2 functions:
intersect(list of strings, list of strings) list of strings
intersect(set of strings, set of strings) set of strings

This implementation is not ideal; instead, the following functions are proposed to replace them:

1. a set of strings function for any number of list/set of strings arguments which produces a set of strings from all items of the given arguments (similar to the list of strings function):
   set of strings(list of strings..., set of strings...) set of strings

2. a intersect function for two items, both of which may be either a list of strings or a set of strings; the intersect in any case returns a set of strings. It can be implemented as single expression (and piece of code) which works for all the cases:
   intersect(list/set of strings, list/set of strings) set of strings