Tree building: eliminate needless memory copying.

Again, sorry for these numerous tweaks I am doing post factum. It would be otherwise difficult, even impossible, to imagine the final picture in the beginning of the whole PR sequence, before having all these iterations done.

By now I've seen 1585ms as lower result for building 100 000 annotations on iPhone 4, compared to previous best - 1,7 seconds.

It is not an obsession with performance ;) but rather the practice exercises on a stuff I am preparing now for my interview.

Cheers!