Dispose procedure

  DECLARATION:  Dispose(var p : Pointer)
                Dispose(P, Destruct)       (* dispose of object *)

      PURPOSE:  Disposes a dynamic variable

         UNIT:  System

      REMARKS:  p is a pointer variable of any pointer type that
                was previously assigned by the New procedure or has
                been assigned a valid value by an assignment statement.
                Dispose releases the variable and space on the heap it
                had occupied.  After a call to Dispose, the value of p
                becomes undefined and an error will result if you attempt
                to reference the pointer variable p.

                When used to dispose of a dynamic object, the effect of
                the extended syntax is the same as executing:

                P^.Destruct;
                Dispose(P);

 RESTRICTIONS:  If p does not point to a memory region in the heap,
                a run-time error occurs.