clean up comprehensions to bring them closer to generator expressions, make their...

clean up comprehensions to bring them closer to generator expressions, make their scoping behaviour configurable
remove optimisations for set([...]) and dict([...]) as they do not take side-effects into account: unhashable items lead to pre-mature exit from the loop
instead, transform set(genexp), list(genexp) and dict(genexp) into inlined comprehensions that do not leak loop variables