Implement std::num::Zero for bool

[bstrie]

This currently fails:

#[deriving(Zero)]
struct Foo {
    bar: bool
}

fn main() {}

zb.rs:1:11: 1:15 error: failed to find an implementation of trait std::num::Zero for bool
zb.rs:1 #[deriving(Zero)]
                   ^~~~

Without such an implementation, it's impossible to use #[deriving(Zero)] on structs that contain bools. I presume that the proper "zero" value is false.