refactor(tvix/eval): Encapsulate Value::Attrs construction

Factor out the construction of Value::Attrs (including the Rc) into a
new `attrs` constructor function, to abstract away the presence of the
Rc itself.

Change-Id: I42fd4c3841e1db368db999ddd651277ff995f025
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6892
Autosubmit: grfn <grfn@gws.fyi>
Reviewed-by: sterni <sternenseemann@systemli.org>
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/eval/src/value/mod.rs

@@ -115,6 +115,14 @@ impl<'a> Deref for ForceResult<'a> {
     }
 }
 
+/// Constructors
+impl Value {
+    /// Construct a [`Value::Attrs`] from a [`NixAttrs`].
+    pub fn attrs(attrs: NixAttrs) -> Self {
+        Self::Attrs(Rc::new(attrs))
+    }
+}
+
 impl Value {
     /// Coerce a `Value` to a string. See `CoercionKind` for a rundown of what
     /// input types are accepted under what circumstances.