Rule: No unused variables (Closes #8)

Closes #8
I can't say that I'm happy with this code, it affects performance way too much (increased performance test from 1.9s to 3s for me). But I already re-wrote this code 3 times from the scratch and I'm getting a bit tired looking at it. Maybe somebody else have ideas how to improve on this? I'm also pretty sure that I'm not thinking about all corner cases.