feat(tvix/eval): implement OpPopWith

Change-Id: I752d9428a73f893741746e9d5b79e8d8d570bb81
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6219
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>
Reviewed-by: grfn <grfn@gws.fyi>

tvix/eval/src/opcode.rs

@@ -54,6 +54,7 @@ pub enum OpCode {
 
     // `with`-handling
     OpPushWith(usize),
+    OpPopWith,
 
     // Lists
     OpList(usize),

tvix/eval/src/vm.rs

@@ -282,6 +282,9 @@ impl VM {
                 }
 
                 OpCode::OpPushWith(idx) => self.with_stack.push(idx),
+                OpCode::OpPopWith => {
+                    self.with_stack.pop();
+                }
             }
 
             #[cfg(feature = "disassembler")]