Math: allow using "^" in place of "**" in calc

`^` is a very common symbol for talking about exponents (as in `2^2`) outside of programming, so it might make sense to allow that in `calc` along with `**`. 

Since the 'bitwise exclusive or' operator (`^` in Python) isn't available in `calc` anyways (it is in `icalc`), this might be useful for user-friendliness. It isn't very obvious that `**` should be used instead of `^` for exponents anyways.