Implement PartialOrd manually for BitMexPriceEventId

This is important because deriving `PartialOrd` means that it looks at
all the struct's fields in order. For this type we want to compare the
`timestamp`, but do not care about the `digits`! More importantly,
someone could unintentionally change this behaviour by reordering the
fields.

We also remove the `Ord` derived implementation because:

- It's unused.
- Deriving `Ord` when manually implementing `PartialOrd` is a very bad
  idea[1].

[1]: rust-lang/rust-clippy#1621.

Co-authored-by: Daniel Karzel <daniel.karzel@coblox.tech>

model/src/olivia.rs

@@ -45,9 +45,7 @@ pub struct Attestation {
     pub scalars: Vec<SecretKey>,
 }
 
-#[derive(
-    Debug, Clone, Copy, SerializeDisplay, DeserializeFromStr, PartialEq, Eq, Hash, PartialOrd, Ord,
-)]
+#[derive(Debug, Clone, Copy, SerializeDisplay, DeserializeFromStr, PartialEq, Eq, Hash)]
 pub struct BitMexPriceEventId {
     /// The timestamp this price event refers to.
     timestamp: OffsetDateTime,
@@ -128,6 +126,12 @@ impl FromStr for BitMexPriceEventId {
     }
 }
 
+impl PartialOrd for BitMexPriceEventId {
+    fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
+        self.timestamp.partial_cmp(&other.timestamp)
+    }
+}
+
 impl From<Announcement> for maia_core::Announcement {
     fn from(announcement: Announcement) -> Self {
         maia_core::Announcement {