Allow genexprs in @cfunc functions.

[memeplex]

This fails to compile:

@cython.cfunc
def f():
    ','.join(x for x in '123')

The problem is that the cfunc directive forces an AdjustDefByDirectives visit. This is ok for the f node but not for the genexpr DefNode (i.e. node.body.stats[0].expr.args[0].def_node, where node is the f node) which inherits the cfunc directive from f and is also (but wrongly) transformed to a CFuncDefNode.

I'm not sure whether there is a valid use case for inheriting cfunc or not. The current implementation just set the visitor directives attribute to the directives in a CompilerDirectivesNode, so the same directives are active for the entire recursive descent. An alternative is to pop the directive after applying it, in cases for which directive inheritance is undesirable. This PR does what I've just described, but only for cfunc. I consider it a dirty hack until more general criteria are discussed and adopted (for example, it seems inconsistent to me doing this for cfunc and not for ccall).

[scoder]

This doesn't look like the best fix, but
a) this is definitely a bug, thanks for your investigation
b) it's even more general as it also applies to other "directives" that shouldn't get inherited