`IN` matching rules for taxonomies (and perhaps other parameters)

[goblindegook]

We need to add IN operator support for taxonomies.

We should consider a more reusable approach to the one we're using, like for instance turning a single IN rule into an array of multiple == rules. This would make the IN operator work for pretty much any parameter type you could throw at it.

[goblindegook]

OTOH, composing rules while splitting a IN rule into multiple == would make it very difficult to generate the final array.