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

fix scoping rules for comprehensions and inlined generator expressions by injecting a separate scope instance

I noticed two failures to compile that I think are bugs:
 - "ctypedef Foo[int] Bar" gives a syntax error (i.e., there doesn't seem to be any way to typedef a templated type).
 - Defining a struct in a namespace and trying to convert it to a Python dict doesn't work. Instead you get nasty "__pyx_convert__to_py_THENAMESPACE" errors from the C++ compiler (it doesn't mangle the cname properly and tries to define __pyx_convert__to_py_THENAMESPACE::THESTRUCT).

I made a patch to *add tests* for the two bugs (not fix them).