new lint: variable scope that can be reduced

[matthiaskrgr]

inspired by cppcheck

It could warn about code like this:

fn do_something(is_true: bool, word: String) -> String {
    let mut s = String::from("hello world");
    if is_true {
        s.push_str(&word);
        s
    } else {
        word
    }
}

where the scope of s can be savely reduced to span across the true branch of the if:

fn do_something(is_true: bool, word: String) -> String {
    if is_true {
      let mut s = String::from("hello world");  
      s.push_str(&word);
      s
    } else {
        word
    }
} 

[killercup]

Either assumes or needs to assert that construction or destruction of s doesn't have side effects.