opr: Add a red/black tree implementation

opr: Add a red/black tree implementation

Add an implementation of red/black trees to our runtime library.

This is originally derived from the FreeBSD macro-based rbtree
implementation, but is heavily reworked to not use macros, to improve
legibility, and to favour speed over structure compactness.

A test suite is provided in tests/opr/

Change-Id: I123209d3f89b5f8c1b85d1e5cd7d1d650ccc68ed
Reviewed-on: http://gerrit.openafs.org/5838
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>