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/value/function.rs

@@ -51,7 +51,7 @@ pub struct Lambda {
     /// Number of upvalues which the code in this Lambda closes
     /// over, and which need to be initialised at
     /// runtime.  Information about the variables is emitted using
-    /// data-carrying opcodes (see [`OpCode::DataLocalIdx`]).
+    /// data-carrying opcodes (see [`OpCode::DataStackIdx`]).
     pub(crate) upvalue_count: usize,
     pub(crate) formals: Option<Formals>,
 }