Introducing a debug mode to the QTL similar to other STL implementations

A lot of STL implementations have a "debug mode" which eases debugging, for example:

1) sort(first, last): both iterators must be from the same container and first <= last
2) dereferencing of iterators is only allowed for valid iterators
3) binary_search(first, last, value): first and last must fulfill 1) and [first,last] must be a sorted range

Something like this could be useful in Qt too.