AdvIncSubtensor1 - better error message in make_node when incrementing a selection of lower rank, and test of that error condition

env.

This used to make pickling of scan impossible. As I've broken the pickling
mechanism several times before, I've added a test that checks if scan is
still picklable. I'm not sure is the best way of doing it though ..

review.

As before, I would suggest reviewing just the cython code and not the C code
since is generated, and hence hard to impossible to review.

while)

Cython can not handle negative indices ( because of a derictive at the
begining of the function that says not to do boundschecks). This is used to
make the code faster, so this workaround is better then removing the
directive.

scan)