Support #[autoimpl] for traits with non-generic targets

[dhardy]

This allows some (possibly rather useless) impls like this:

struct MyVec {
    vec: Vec<()>,
}

#[autoimpl(for MyVec using self.vec)]
trait IsEmpty {
    fn is_empty(&self) -> bool;
}

impl<T> IsEmpty for Vec<T> {
    fn is_empty(&self) -> bool {
        Vec::is_empty(self)
    }
}

Of course, this should be done using #[autoimpl(IsEmpty using self.vec)] on struct MyVec, but without writing a custom extension to impl-tools-lib specifically for the IsEmpty trait that isn't possible.