Final version of the lab (I hope...).

I fixed the weird bug that was making the operator disappear.
It turns out that I needed to add 1 to the malloc calls in main
to make space for the NULL terminator in my strings (duh). Now
everything works, except for a small bug. Since I use a long
to store the data for print_decimal, it can't correcly do subtraction
when the word size is large (I used 64). I tried a quick change to
long long, but that did not help... I'm sure I could get it working, but
really I'm to lazy. Future me, you should fix it.

I'M DONE!