Add const ZERO for use in statics

[ExplodingWaffle]

Wanted to initialise an Instant in a static, but I couldn't because zero() is a (non-const) fn. AFAIK you can't have const fns in traits, so no constifying zero() - this is the next best thing, I think.
Since from_ticks() in fugit is const, implementing this is the same as zero() -

const ZERO: Self::Instant = Self::Instant::from_ticks(0);