feat(tvix/eval): implement attrset update (//) operator

The underlying implementation does a few tricks based on which pair of
attrset representations is encountered.

Particularly the effect of short-circuiting the empty cases might be
relevant in nixpkgs/NixOS, due to the use of lib.optionalAttrs.

Change-Id: I22b978b1c69af12926489a71087c6a6219c012f3
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6140
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

tvix/eval/src/value/mod.rs

@@ -71,6 +71,16 @@ impl Value {
             }),
         }
     }
+
+    pub fn as_attrs(self) -> EvalResult<Rc<NixAttrs>> {
+        match self {
+            Value::Attrs(s) => Ok(s),
+            other => Err(Error::TypeError {
+                expected: "set",
+                actual: other.type_of(),
+            }),
+        }
+    }
 }
 
 impl Display for Value {