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),