Slicing: Implement backwards slicing.

[romanofski]

Backwards slciing needs to be implemented to provide full slcing

[donkirkby]

This is the feature I was hoping for when I came looking for a Python slicer.

[romanofski]

I can imagine, yet I'm currently not spending any time on this actively coding, but rather on education. The current implementation is deficient in a way of correctly building a dependency graph. From there adding the backwards slicing shouldn't be much of a challenge.