Rule request: prefer implicit returns when possible

[marcelofabri]

From raywenderlich.com Swift Style Guide

---

For single-expression closures where the context is clear, use implicit returns:

attendeeList.sort { a, b in
    a > b
}

[jpsim]

related to #70 (only use positional arguments in "simple" closures)

[marcelofabri]

While this is related to #70, it's a bit different since we don't need to decide what "simple" means. Implicit returns are only allowed on single-expression closures.

[jpsim]

Although it's not always easy to determine if something is a single expression or not. It's not exactly directly determined by number of lines or presence of semicolons.