support compound indexes as a vector

[thedavidmeister]

rethinkdb lets us define compound indexes without needing a function if we pass an array of values.

https://www.rethinkdb.com/docs/secondary-indexes/javascript/

It would be good if we could do something similar like:

(r/index-create (r/table "users") "full_name" ["first_name" "last_name"])

[apa512]

The official clients convert the array to a function but I guess we could do the same (just pointing out that RethinkDB actually only supports field name or function in case someone else wants to add this).