Extend single_match to handle return

[mrmonday]

In the following code:

enum Foo {
    Bar(u8), Baz
}

fn main() {
    match Foo::Bar(3) {
        Foo::Bar(_) => { /* do something */ },
        _ => { return; }
    }
}

Clippy produces no warning, but it could produce the single_match warning instead.

[mcarton]

It was decided in #327 to limit the lint to enum we know will never be extended.
But we could have another allow-by-default lint for that.

[camsteffen]

This is now covered by single_match_else