refactor(tvix/eval): remove unnecessary clones in compiler

There's basically nothing that needs *ownership* of an AST
node (which is just a little box full of references to other things
anyways), so we can thread this through as references all the way.

Change-Id: I35a1348a50c0e8e07d51dfc18847829379166fbf
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6853
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>

corp/tvixbolt/src/main.rs

@@ -254,7 +254,7 @@ fn eval(trace: bool, code: &str) -> Output {
     let mut compilation_observer = DisassemblingObserver::new(codemap.clone(), &mut out.bytecode);
 
     let result = tvix_eval::compile(
-        root_expr,
+        &root_expr,
         Some("/nixbolt".into()),
         file.clone(),
         tvix_eval::global_builtins(),