fix(tvix/eval): address all current clippy lints

Change-Id: I758fc4f3b9078de7ca6228a75a4351c3e085c4cf
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6272
Reviewed-by: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI

tvix/eval/src/opcode.rs

@@ -7,6 +7,7 @@ pub struct ConstantIdx(pub usize);
 #[derive(Clone, Copy, Debug)]
 pub struct CodeIdx(pub usize);
 
+#[allow(clippy::enum_variant_names)]
 #[warn(variant_size_differences)]
 #[derive(Clone, Copy, Debug)]
 pub enum OpCode {