V4 IR: change datastructure of the worklist used by type inference.

No sets, no cry, and no mallocs of QHashNodes.

QBitArray is not used on purpose: good STL implementations specialize
std::find for std::vector<bool>, with a significant speed gain.

Change-Id: Ic986bbd746e96eb494496f0508acb17318f4f0fb
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>