Add support for the combinatoric operators "comb" and "perm"

Heavily inspired in !3 (closed).

Your code seems correct, it was just in the wrong place, as we've left the operations file only for the original mathml2 spec. This is something that could be reviewed but I think that this also makes it a bit easier to work with.

I did not include the complex variant of the change because it seemed to me that it was just taking the real part, please tell me if there's something I missed.