refactor(tvix/eval): remove code and location from struct

Instead, it's passed in the evaluate/compile_only functions, which feels
more naturally. It lets us set up the Evaluation struct long before
we actually feed it with data to evaluate.

Now that Evaluation::new() would be accepting an empty list of
arguments, we can simply implement Default, making things a bit more
idiomatic.

Change-Id: I4369658634909a0c504fdffa18242a130daa0239
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10475
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>
Autosubmit: flokli <flokli@flokli.de>

tvix/eval/tests/nix_oracle.rs

@@ -51,12 +51,12 @@ fn nix_eval(expr: &str, strictness: Strictness) -> String {
 #[track_caller]
 fn compare_eval(expr: &str, strictness: Strictness) {
     let nix_result = nix_eval(expr, strictness);
-    let mut eval = tvix_eval::Evaluation::new(expr, None);
+    let mut eval = tvix_eval::Evaluation::default();
     eval.strict = matches!(strictness, Strictness::Strict);
     eval.io_handle = Box::new(tvix_eval::StdIO);
 
     let tvix_result = eval
-        .evaluate()
+        .evaluate(expr, None)
         .value
         .expect("tvix evaluation should succeed")
         .to_string();