Fix reassignment of the keys in delete operation

[w8r]

This is horrible, the nodes are supposed to be immutable.

[simonepri]

Can you state it on the readme while this is getting fixed?

[w8r]

yup, in the meanwhile you can consider using w8r/splay-tree, it's even faster

[w8r]

and has no such problem

[simonepri]

@w8r Thanks! I've already solved most of the problems anyway. I saw that that package has also the api to solve this https://github.com/w8r/avl/issues/21.