Using your special vector of ints from assignment 3, create a basic
super-calculator. It should be able to perform simple arithmetic 
operations on positive integeres with an unlimited number of digits.
You should create a class which represents big ints with at least
the following methods and constructors:

   bigint(string val) - bigint a("12345678987654321") initialises a 
                        to 12345678987654321
   bigint(int val)    - bigint b(357) initialises b to 357
   bigint()           - initialises to zero
   print()            - a.print() prints the value of a neatly
   copy(bigint x)     - a.copy(b) makes a equal to b - assignment
   add(bigint x)      - a.add(b) makes a equal to a+b
   subtract(bigint x) - a.subtract(b) makes a equal to a-b, it is not
                        required to work if the result is negative
   compare(bigint x)  - returns the int -1 is a<b, zero if a==b, and
                        1 if a>b.
   multiply(int x)    - a.multiply(b) makes a equal to a*b where b is
                        a smallish ordinary int
   shift(int x)       - a.shift(b) multiplies a by 10 to the power of b.
                        In other words, it adds b zeros to the end.