Implement "Use last" lint

[HarrisonMc555]

Closes #3673

This lint checks the use of x.get(x.len() - 1) and suggests x.last() (where x is a Vec).

There's at least one more thing that needs to happen here. I believe I am correctly checking most of the scenarios and avoiding false positives. However, if different Vecs are used for the x.get and y.len, then it will emit a false positive. In other words, I need to check to make sure the Vecs are the same.

Also, a lot of these commits were temporary and not helpful to the project history...am I supposed to squash on merge? If so, how do I do that?

changelog: New lint: get_last_with_len