Fix: unit structures should have 1 possible values, not 0

[aarkegz]

A unit struct with Enumerable derived now enumerate 0 values.

#[derive(Enumerable, Copy, Clone)]
struct A;

fn main() {
    let all: Vec<_> = A::enumerator().collect();

    println!("{:?}", all.len()) // 0
}

However, it's not correct, since A{} is undoubtedly a legal expression.