Implement total ord for Key

[mitsuhiko]

Fixes potential crashes on Rust 1.81.

[mitsuhiko]

Unfortuately rust-lang/rust#106418 does not exist yet, so std::mem::discriminant cannot be used.

[orlp]

@mitsuhiko I don't believe an explicit discriminant value was necessary here, I believe this would've sufficed:

if let (F64(l), F64(r)) = (self, other) {
    f64_total_cmp(*l, *r)
} else {
    self.partial_cmp(other).unwrap()
}

EDIT: never mind that's not true due to our overly aggressive requirements on Ord. We currently require
partial_cmp(a, b) == Some(cmp(a, b)) instead of the more relaxed partial_cmp(a, b) being Some implying equality to cmp(a, b).

You could still apply the above pattern but it's probably not worth it, you'd need a wrapper dummy struct just to derive PartialOrd on...