This is a big one:

* cleanup grammar; unifies listcomp/genexp grammar
which means that [x for x in 1, 2] is no longer valid

* cleanup comprehension compiling code (unifies all AST
code for the three comprehensions and most of the
compile.c code)

* add set comprehensions

This patch modifies list comprehensions to be
implemented more like generator expressions: in a
separate function, which means that the loop variables
will not leak any more.