multiplication function and small updates

[veronewra]

Hi andrew! You probably havent looked at this repository in a while, I've made a few changes: -renamed the master branch to "main" since that is githubs default now -added a basic workflow (your program currently cant build because I think some of your rust syntax isnt up to date) -added a function for multiplying complex polynomials, along with a test for it. I wasn't sure if I should update the CPolynomial struct to include a vector for degrees, since it would mess with a lot of stuff I decided not to for now