The `birth_rotation` attribute replaces `birth_mode`

The `birth_rotation` attribute replaces `birth_mode`

The `birth_mode` was not sufficient for properly unrotate the brother of
a removed node. The `birth_rotation` stores the rotation applied to the
brother subtree when the node was inserted.

This should help to enforce that (L + n) - n = L in every possible
cases. Which means that adding a node and removing it should leave the
layout unchanged.