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/serde/src/de.rs

@@ -46,12 +46,12 @@ where
     F: FnOnce(&mut Evaluation),
 {
     // First step is to evaluate the Nix code ...
-    let mut eval = Evaluation::new(src, None);
+    let mut eval = Evaluation::default();
     config(&mut eval);
 
     eval.strict = true;
     let source = eval.source_map();
-    let result = eval.evaluate();
+    let result = eval.evaluate(src, None);
 
     if !result.errors.is_empty() {
         return Err(Error::NixErrors {