support setting language level (2 or 3) for source code - currently configures string literals and print()

disable tuple(genexpr) as it has a non-obvious negative effect on memory usage - better let users write tuple([...]) explicitly

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