[Patch] RBTree iteration interface improvement

Hello

While working on some new feature for PostgreSQL (which is still in
development and is irrelevant in this context) I noticed that current
RBTree implementation interface is following:

```
extern void rb_begin_iterate(RBTree *rb, RBOrderControl ctrl);
extern RBNode *rb_iterate(RBTree *rb);
```

As you see it doesn't allow to do multiple iterations over a single
tree. I believe it's a major flaw for a general-purpose container.

Proposed patch introduces a new iteration interface that doesn't has
such a flaw. Naturally I wrote some unit tests, but I was not sure where
exactly to place them in PostgreSQL source code. For now I've just
uploaded them to GitHub:

https://github.com/afiskon/c-algorithms-examples/blob/master/rbtree_example.c

According to these tests new implementation works as fast as current
implementation and produces exactly same results.

I didn't dare to remove current interface since in theory some
extensions can use it. Still I believe such a move is worth considering.

--
Best regards,
Aleksander Alekseev