ps stack: Manage a stack like in Postscript

This class can be used to manage a LIFO stack (Last In First Out) of values like in the Postscript language.

It implements most of the Postscript stack operations except for mark and cleartomark.

The class can push and pop values from the stack, get the value at the top of the stack or at a given position, get the count of values, exchange the position of two values, duplicate the value at the top of the stack, roll the stack values by a given number of positions.