* Add RB_FOREACH_FROM() which continues traversal *at*
the y-node provided. There is no pre-increment.
* Nuke RB_FOREACH_SAFE as it was buggy; it would omit the final node.
* Replace RB_FOREACH_SAFE() with a working implementation
derived from RB_FOREACH_FROM().
The key observation is that we now only check the loop-control
variable, but still cache the next member pointer.
* Add RB_FOREACH_REVERSE_FROM() which continues backwards
traversal *at* the y-node provided. There is no pre-increment.
Typically this is used to back out of allocations made
whilst walking an RB-tree.
* Add RB_FOREACH_REVERSE_SAFE() which performs insertion and
deletion safe backwards traversal.
7dc8e87695