PartialOrd fix for ArchivedVec.

[gz]

Without it, I couldn't get e.g., this to compile since it couldn't compare Vec with ArchivedVec:

#[derive(Debug, Clone, Archive, Serialize, Deserialize)]
#[archive_attr(derive(Clone, Ord, Eq, PartialEq, PartialOrd))]
#[archive(compare(PartialEq, PartialOrd))]
pub struct PersonalNetworkGkgEntry {
    pub people: Vec<String>,
}

(FYI I hope that this partial_ord impl is actually correct since I haven't written any PartialOrd for slices/vectors before so definitely good to double check :O)

[djkoloski]

Yep, this looks about the same as the PartialOrd impl for slice.