refactor(tvix/eval): rename Opcode::DataLocalIdx to DataStackIdx

It is very confusing that this opcode is called DataLocalIdx, but it
carries a StackIdx rather than a LocalIdx.  It seems like this
really ought to be called DataStackIdx, but maybe I've
misunderstood; if so please explain it to me.

Change-Id: I91f6ffa759412beef0b91d3c19ec0d873fe51b99
Signed-off-by: Adam Joseph <adam@westernsemico.com>
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7088
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/eval/src/compiler/mod.rs

@@ -1031,7 +1031,7 @@ impl Compiler<'_> {
                         if slot == idx {
                             self.scope_mut().mark_must_thunk(slot);
                         }
-                        self.push_op(OpCode::DataLocalIdx(stack_idx), &upvalue.span);
+                        self.push_op(OpCode::DataStackIdx(stack_idx), &upvalue.span);
                     }
                 }