* associate with more data types
  * ACL?
  * BLOB?
  * BOOL
  * BYTES?
  * DURATION
  * HEADER, HTTP?
  * IP?
  * REAL
  * STEVEDORE?
  * TIME?

* QP_Insert() with rotation, so that ordered inserts are not required

* extend "secondary regex" support
  * method .backref()
  * caseless match option
  * expose pcre limit params for regexen?
For all of these, we'd have to use VRE_* rather than VRT_VRE_*

* search algorithm for .matched() ?
  * See the XXX comment in the code.
