operator overloading for PilePointer

[furesoft]

add operators to PilePointer like

!ptr dereferencing

[zhabis]

Would need to be on Piled, as PilePointer does not contain what pile the pointer is from

[zhabis]

Added binary operator & in commit c079e71:

//PilePointer.cs
public static object operator &(IPile pile, PilePointer ptr)
{
   return pile.NonNull(nameof(pile)).Get(ptr);//->     var x = pile&ptr;
}

//Usage:

//using Get
var obj = pile.Get(ptr);

//using operator
var obj = pile & ptr;