refactor(tvix/value): encapsulate attrset logic within value::attrs

The internal optimisations of the set representation were previously
leaking into the VM, which is highly undesirable.

Keeping it encapsulated allows us to do additional optimisations
within value::attrs without being concerned about its use in the VM.

Change-Id: I7e7020bb0983b9d355d3db747b049b2faa60131f
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6108
Reviewed-by: eta <tvl@eta.st>
Tested-by: BuildkiteCI

tvix/eval/src/value/mod.rs

@@ -25,6 +25,7 @@ pub enum Value {
     // Internal values that, while they technically exist at runtime,
     // are never returned to or created directly by users.
     AttrPath(Vec<NixString>),
+    Blackhole,
 }
 
 impl Value {
@@ -47,7 +48,7 @@ impl Value {
             Value::List(_) => "list",
 
             // Internal types
-            Value::AttrPath(_) => "internal",
+            Value::AttrPath(_) | Value::Blackhole => "internal",
         }
     }
 
@@ -85,7 +86,7 @@ impl Display for Value {
             Value::List(list) => list.fmt(f),
 
             // internal types
-            Value::AttrPath(_) => f.write_str("internal"),
+            Value::AttrPath(_) | Value::Blackhole => f.write_str("internal"),
         }
     }
 }