Add arithmetic to categoricals?
I'm not entirely sure why (perhaps just the trouble of implementing it), but categoricals do not currently support arithmetic:

> In contrast to statistical categorical variables, categorical data might have an order (e.g. ‘strongly agree’ vs ‘agree’ or ‘first observation’ vs. ‘second observation’), but numerical operations (additions, divisions, ...) are not possible.

Similarly to string operations (#8627), arithmetic with scalars could very efficient transform categoricals into new categoricals.

In contrast, array + array operations should probably just return a normal array.

CC @JanSchultz @jreback 
