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/vm.rs

@@ -117,8 +117,17 @@ impl VM {
                 OpCode::OpNull => self.push(Value::Null),
                 OpCode::OpTrue => self.push(Value::Bool(true)),
                 OpCode::OpFalse => self.push(Value::Bool(false)),
+
                 OpCode::OpAttrs(count) => self.run_attrset(count)?,
                 OpCode::OpAttrPath(count) => self.run_attr_path(count)?,
+
+                OpCode::OpAttrsUpdate => {
+                    let rhs = self.pop().as_attrs()?;
+                    let lhs = self.pop().as_attrs()?;
+
+                    self.push(Value::Attrs(Rc::new(lhs.update(&rhs))))
+                }
+
                 OpCode::OpList(count) => self.run_list(count)?,
                 OpCode::OpInterpolate(count) => self.run_interpolate(count)?,
             }