Adding Irrationals to Expression works.
2 * pi crashed. Now yields 2*pi
(pi + 6) + pi crashed. Now yields 2*pi + 6.
Adding negative rationals or irrationals also works. However, if the
result is 0 then 0 will either stay there or the program will crash.

Had to modify the coefficient stuff a bit in IrrationalNumber and then
update Add to deal with the adding.
Also, updated Multiply to handle IrrationalNumbers and RationalNumbers
only. (Adding Expression support is next)
IrrationalNumber still needs Value* coefficient support. I know it's
going to be a bitch but it's the only way.
We can probably keep the coefficient code with integers. Just add the
other stuff around that.