Implement `as` operator overload trait

[brendanzab]

Proposed trait for inclusion in core::ops:

#[lang="cast"]
trait Cast<T> {
    fn cast(&self) -> T;
}

Example implementation:

impl Cast<f32> for float { fn cast(&self) -> f32 { self as f32 } }
impl Cast<int> for f32   { fn cast(&self) -> int { self as int } }

impl<T: Cast<U>, U> Cast<Vec3<U>> for Vec3<T> {
    fn cast(&self) -> Vec3<U> {
        Vec3 { x: self.x as U, y: self.y as U, z: self.z as U }
    }
}

type Vec3f   = Vec3<float>;
type Vec3f32 = Vec3<f32>;
type Vec3i   = Vec3<int>;

fn main() {
    let a = Vec3 { x: 1f, y: 1f, z: 1f };
    let b = a as Vec3f32;
    let c = b as Vec3i;
}

Perhaps there could be a deriving attribute as well:

#[deriving(Cast)]
struct Vec3<T> { x: T, y: T, z: T }

[emberian]

See #7080

[emberian]

(closing as dup)