Add parallel query support functions for assorted aggregates.

This lets us use parallel aggregate for a variety of useful cases
that didn't work before, like sum(int8), sum(numeric), several
versions of avg(), and various other functions.

Add some regression tests, as well, testing the general sanity of
these and future catalog entries.

David Rowley, reviewed by Tomas Vondra, with a few further changes
by me.