Fixed documentation about FAST_COMPILE mode to match its actual implement

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)

All these changes have been generated by cython, and as such I don't think
they need to be reviewed but rather reviewing the cython code should be more
then sufficient.

In fact, it could hid problem that where not expected.