AVL library update

[Adrian Hey]

Hello,

I've put a new version 2.3 of Data.Tree.AVL here..
 http://homepages.nildram.co.uk/~ahey/HLibs/Data.Tree.AVL/

Changes in this version include..
* Added genPush' and genPushMaybe' functions which provide
  strictness control which was missing from earlier versions.
* Added symmetric difference to the set functions
* I decided to use the prefix "assert" for partial function
  names. So there has been a bit of renaming (old names are
  still OK but deprecated, and I might have missed a few :-)

Regards
--
Adrian Hey