Giant numbers part 2.

Improve you giant number calculator so that it can handle negative
numbers and multiplication and division.

Then give it an interactive user interface that allows the user to
enter expressions in Reverse Polish Notation, then evaluates them.

A sample session might look something like this:

input:  123 2 + 50 25 25 + + *

output: 12500

input:  99999999999999999999999999999999999999999999999 2 * 1 -

output: 199999999999999999999999999999999999999999999997

input:  -3 -4 *

output: 12