Make the cosine and sine expression mathematically rigorous

[CaptainChicky]

As it currently stands, the cosine and sine trig expression is not mathematically rigorous. I've included a limit before the cosine and sine expression, which randomly chooses an integer between -10 and 40 to take it to.

I've included the fix for 21 in https://github.com/enjeck/num2math/pull/22 here as well.