Big speedup in GTree

[Maurizio Monge <maurizio monge gmail com>]

Hi,
my implementation of threaded tree for glib is now something
much more similar to a rewrite than to a patch, but i now have got
a 2x speedup in g_tree_insert and 2.5x in g_tree_remove when 
using a simple compare function (in real word with a complicated
compare function the speedup will be smaller), and it is now possible
to modify the tree while traversing it (using a sort of iterators).
See Bug #169285 for a snapshot of the code and a small benchmark.
Let me not if you find any bug.

Maurizio Monge