fix(tvix/value): explicitly delegate Ord to &str representation

I assumed that `Ord` is a marker trait like `Eq`, but it actually has
a member. Without this ordering was incoherent.

Change-Id: Id37cbdf333daf748d29b85243046c7e061b1ce29
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6139
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/value/string.rs

@@ -4,7 +4,7 @@ use std::{borrow::Cow, fmt::Display};
 /// This module implements Nix language strings and their different
 /// backing implementations.
 
-#[derive(Clone, Debug, Eq, Ord)]
+#[derive(Clone, Debug)]
 pub enum NixString {
     Static(&'static str),
     Heap(String),
@@ -16,12 +16,20 @@ impl PartialEq for NixString {
     }
 }
 
+impl Eq for NixString {}
+
 impl PartialOrd for NixString {
     fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
         self.as_str().partial_cmp(other.as_str())
     }
 }
 
+impl Ord for NixString {
+    fn cmp(&self, other: &Self) -> std::cmp::Ordering {
+        self.as_str().cmp(other.as_str())
+    }
+}
+
 impl From<&'static str> for NixString {
     fn from(s: &'static str) -> Self {
         NixString::Static(s)