Add Return option for ctrl_flow

[livia01px2019]

Create a new CtrlSink struct that is either a CallSite or a Return so that we can capture control flows to returns, e.g. in this example:

fn encrypt_broken_basic(user_data: &PrivateUserData) -> bool {
    if user_data.key.len() > 10 {
        return true;
    }
    return false;
}

We want to express the ctrl flow from user_data to the return.