chore(tvix/eval): add variant_size_differences warning to Value

Change-Id: I2f4719a81512a9a970edf22390afed490125bda7
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6182
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>

tvix/eval/src/value/mod.rs

@@ -12,6 +12,7 @@ pub use attrs::NixAttrs;
 pub use list::NixList;
 pub use string::NixString;
 
+#[warn(variant_size_differences)]
 #[derive(Clone, Debug)]
 pub enum Value {
     Null,