Red black trees

[Walter Bright]

clayasaurus wrote:
> I'd also like to add, that if you do give me the heads up that you would 
> like to see this to phobos, give me time to...

I'd like to:

1) change RedBlackTree to take two type arguments: Key and Value types. 
The user needn't see the Node type at all.

2) enough unittest cases so that running it with -cov gives 100% coverage.

3) the following iterators: opApply (forward), opApplyReverse (reverse).