intbitset: initialization from iterator

• Fixes initialization from an iterator, by peeking for the iterator protocol signature in the object used to initialize an intbitset data structure thus avoiding calling max() in case pre-allocation is needed, which would consume the whole iterator before allowing the data to be extracted (and that would crash when max() is called on an empty iterator). (closes #1698)

Signed-off-by: Samuele Kaplun <samuele.kaplun@cern.ch>