Fix code style of condintionals with logic flipping

* Replace `if not a` with `unless a`
* Replace `if not a and not b` with `unless a or b`
* Replace `if not a or not b` with `unless a and b`
Fix code style of condintionals not in if/unless statements

The same thing as for if/unless statements but for conditional
assignments:
* `a ||= b` -> `a or= b`
* `a = b || c` -> `a = b or c`
* `a = not b && not c` -> `a = not (b or c)`
Fix code style of while loops
Use string interpolation instead of concatentation when possible
Remove trailing spaces
Replace if checks with existential operator when possible