refactor(tvix/eval): Builderize Evaluation

Make constructing of a new Evaluation use the builder pattern rather
than setting public mutable fields. This is currently a pure
refactor (no functionality has changed) but has a few advantages:

- We've encapsulated the internals of the fields in Evaluation, meaning
  we can change them without too much breakage of clients
- We have type safety that prevents us from ever changing the fields of
  an Evaluation after it's built (which matters more in a world where we
  reuse Evaluations).

More importantly, this paves the road for doing different things with
the construction of an Evaluation - notably, sharing certain things like
the GlobalsMap across subsequent evaluations in eg the REPL.

Fixes: b/262
Change-Id: I4a27116faac14cdd144fc7c992d14ae095a1aca4
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11956
Tested-by: BuildkiteCI
Autosubmit: aspen <root@gws.fyi>
Reviewed-by: flokli <flokli@flokli.de>

tvix/eval/tests/nix_oracle.rs

@@ -58,10 +58,16 @@ fn nix_eval(expr: &str, strictness: Strictness) -> String {
 #[track_caller]
 #[cfg(feature = "impure")]
 fn compare_eval(expr: &str, strictness: Strictness) {
+    use tvix_eval::EvalIO;
+
     let nix_result = nix_eval(expr, strictness);
-    let mut eval = tvix_eval::Evaluation::new_pure();
-    eval.strict = matches!(strictness, Strictness::Strict);
-    eval.io_handle = Box::new(tvix_eval::StdIO);
+    let mut eval_builder = tvix_eval::Evaluation::builder_pure();
+    if matches!(strictness, Strictness::Strict) {
+        eval_builder = eval_builder.strict();
+    }
+    let eval = eval_builder
+        .io_handle(Box::new(tvix_eval::StdIO) as Box<dyn EvalIO>)
+        .build();
 
     let tvix_result = eval
         .evaluate(expr, None)