#2 interface for split, cmp

[Nathan Hurst]

This patch adds the ability to efficiently split an avl tree around a value.

I'm not entirely happy with the interface, if you pass an element that is in the range of the tree but not in the tree it should split the around that value(perhaps returning None as the centre item), currently it just returns the entire tree.

cmp returns whether an item is in the range of the tree.

Having implemented this, I'm now wondering whether a better interface would be:
get the root node of the tree
pivot the tree around a node
* split the root into (left, node, right)

However, pivot is a bit distasteful, because it unbalances the tree (and any further operations would tend to unpivot the tree)

[Nathan Hurst]