Go crazy with associated constants (ZERO, ONE, etc).

[yoanlcq]

Add meaningful associated constants along with existing functions that are meant as constants.
Sadly this is currently not possible (num-traits's Zero doesn't have ZERO), but I'd prefer not to forget.

impl<T: Zero> Vec4<T> {
    pub fn zero() -> Self { Self::ZERO }
    pub const ZERO: Self = $Vec { $($get: T::ZERO,)+ };
}