Expand the 'reverse-string' analyzer

[ZapAnton]

The current version of the reverse-string analyzer does a very primitive version of the analysis - read the solution AST and compare it with a bunch of prepared ASTs to give a approve_as_optimal or approve_with_comment status.

The prepared ASTs can be viewed in the reverse-string README.

The following work should be done:

• Make the AST analysis more thorough
• Expand the approve solution list
• Make the analyzer return the disapprove status

[ZapAnton]

Future note: currently the following solution is not recognized as approve_as_optimal:

use unicode_segmentation::UnicodeSegmentation;
pub fn reverse(input: &str) -> String {
    UnicodeSegmentation::graphemes(input, true).rev().collect::<String>()
}

This is because the graphemes method is called via the crate name instead of input &str

[HarrisonMc555]

I honestly have no idea if this is the case or not, and I don't have time to look into it right now, but from my experience with clippy there was a distinction between "early" passes and "late" passes...could a "late" pass be in such a format that everything is "desugared" and the function calls would be identical? Just a thought. Not sure if that's even available in the syn crate

P.S. Sorry for being MIA, I started a couple of other projects. I'd like to contribute some more but I also start a new job on Monday so I'm not sure how much time I'll have.