Write functions simulated following operations of Stack ADT with
Link list implementation
Limit size of items in Queue ADT is 20 items
1. newStack(): Creates a new empty stack.
2. isEmpty(*S): Returns a Boolean value indicating if the
stack is empty.
3. size(*S): Returns the number of items in the stack.
4. push(*S,element): Adds the given element to the top of
the stack.
5. pop(*S): Removes and returns the top item of the stack
6. top(*S): Returns an item on top of a non-empty stack
without removing it.