Merge addRe and addI, and mulI and mulRe

[balacij]

    I agree about merging `addRe` and `addI` and hiding.

    The errors are real (pun intended) basically because none of those things should be `Rational`, that seems like a plain hack that should be fixed.

Originally posted by @JacquesCarette in #3131 (comment)

[daijingz]

@balacij Jason, I want to work on this issue. Although previously I did not do any difficult questions. I want to have a try.

The code is at \code\drasil-lang\lib\Language\Drasil\Expr\Class.hs

In my opinion, the first part we need to do is check whether both function types are compatible or not. However, both functions have arbitrary types, so I need to check other function callings first.