Closest search functions & range lookups

Please review my patchset which adds functions to search closest elements in the trees. The functions are useful for at least range lookups.

Overview of the changes:
- some fixes in dict_itor_search()
- code cleanup mostly related to dict_itor_search()
- new functions: dict_search_le(), _lt(), _ge(), _gt() & the same for dict_itor_search
- unit tests for all the new functions & dict_itor_search()