Add compatibility with v1 routing

Following on from #564.

This adds a lot more compatibility than we had by reintroducing the Reflection magic argument matching, and again, is only a stop-gap.

This forms the start of our v1 compatibility, and will be moved once we've ported over to v2.

(Note: this doesn't have full compatibility, as things like `WP_JSON_Server::READABLE | WP_JSON_Server::ACCEPT_JSON` will cast the READABLE constant string to 0. We'll fix this in the real v1, since v2 will be renamed to `WP_API_Server` instead.)