chore(tvix/eval): explicitly set #[repr(transparent)] on wrappers

For representation wrappers that are used to control the visibility of
type internals, this ensures that the wrapper does not increase the
size of the type.

In practice, the optimiser likely does this anyways but it is good to
guarantee it.

Change-Id: Ic6df7d668fe6006dfbd5b6cfcfc2088afa95b810
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6178
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>

tvix/eval/src/value/string.rs

@@ -11,6 +11,7 @@ enum StringRepr {
     Heap(String),
 }
 
+#[repr(transparent)]
 #[derive(Clone, Debug)]
 pub struct NixString(StringRepr);