feat(tvix/eval): construct internal attribute path representation

This is required for constructing nested attribute sets at runtime.

There'll be quite a lot of optimisation potential with this solution
eventually, if it should turn out to be a bottleneck.

This introduces a conceptual change, in that the `Value` enum is now
an enum representing "all runtime values" instead of "all Nix language
types". This makes sense in general, as this type will also contain
Chunk representations etc. which are not exposed to users.

Change-Id: Ic5f72b2a0965b146c6a451efad34c6a81ca1aad8
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6103
Reviewed-by: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI

tvix/eval/src/value/mod.rs

@@ -21,6 +21,10 @@ pub enum Value {
     String(NixString),
     Attrs(Rc<NixAttrs>),
     List(NixList),
+
+    // Internal values that, while they technically exist at runtime,
+    // are never returned to or created directly by users.
+    AttrPath(Vec<NixString>),
 }
 
 impl Value {
@@ -41,6 +45,9 @@ impl Value {
             Value::String(_) => "string",
             Value::Attrs(_) => "set",
             Value::List(_) => "list",
+
+            // Internal types
+            Value::AttrPath(_) => "internal",
         }
     }
 
@@ -76,6 +83,9 @@ impl Display for Value {
             Value::String(s) => s.fmt(f),
             Value::Attrs(attrs) => attrs.fmt(f),
             Value::List(list) => list.fmt(f),
+
+            // internal types
+            Value::AttrPath(_) => f.write_str("internal"),
         }
     }
 }