refactor(tvix/eval): Implement value comparison with a method

Rather than implementing all of the interesting semantics of value
comparison with a macro bound to the VM, implement the bulk of the logic
with a method on Value itself that returns an Ordering, and then use the
macro to implement the comparison against that Ordering. This has no
functional change, but paves the way to implementing lexicographic
comparison of list values, which is supported in the latest version of
upstream nix.

Change-Id: I8af1a020b41577021af5939f5edc160c407d4a9e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7069
Autosubmit: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

tvix/eval/src/value/mod.rs

@@ -1,5 +1,6 @@
 //! This module implements the backing representation of runtime
 //! values in the Nix language.
+use std::cmp::Ordering;
 use std::ops::Deref;
 use std::path::PathBuf;
 use std::rc::Rc;
@@ -348,6 +349,26 @@ impl Value {
         }
     }
 
+    /// Compare `self` against other using (fallible) Nix ordering semantics.
+    pub fn nix_cmp(&self, other: &Self) -> Result<Option<Ordering>, ErrorKind> {
+        match (self, other) {
+            // same types
+            (Value::Integer(i1), Value::Integer(i2)) => Ok(i1.partial_cmp(i2)),
+            (Value::Float(f1), Value::Float(f2)) => Ok(f1.partial_cmp(f2)),
+            (Value::String(s1), Value::String(s2)) => Ok(s1.partial_cmp(s2)),
+
+            // different types
+            (Value::Integer(i1), Value::Float(f2)) => Ok((*i1 as f64).partial_cmp(f2)),
+            (Value::Float(f1), Value::Integer(i2)) => Ok(f1.partial_cmp(&(*i2 as f64))),
+
+            // unsupported types
+            (lhs, rhs) => Err(ErrorKind::Incomparable {
+                lhs: lhs.type_of(),
+                rhs: rhs.type_of(),
+            }),
+        }
+    }
+
     /// Ensure `self` is forced if it is a thunk, and return a reference to the resulting value.
     pub(crate) fn force(&self, vm: &mut VM) -> Result<ForceResult, ErrorKind> {
         match self {