reverse-string: Refactor the lints.rs module

[ZapAnton]

The current implementation of the lint functions in the lints.rs module consists of a great number of the deeply nested if let statements with the intermediate logic inside the condition blocks.

Some block could not be avoided, since the dyn keyword inside the pattern matches is not stable yet, but even without it there is a lot of repetitive and hard to maintain code, which potentially can migrate to the other exercise analyzers.

So it would be great to see if the code could be somehow refactored for better readability/maintainability.