Fix code style of condintionals which aren't 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
Avoid the use of standalone @
Replace regual conditionals with postfix ones when appropriate
Replace loops with comprehensions when appropriate
Replace double quotes with single one when possible
Various minor fixes