Renamed push / pop to insert / take so that it matches the other container types.
Refactoring
Fixed error caused by last renaming.
Added avltree_for_each_postorder_safe()
Don't liked the reverse functions => removed.

Revert "Added *_assert functions: they help to easily verify if a data structure is in a valid state."

This reverts commit da391cde3f54f4d4bc42fde45d023807155c0d2d.
Removed undefined references introduced from last revert.
Refactoring
Removed no longer needed code which was introduced from previous commit.