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>

web/tvixbolt/src/lib.rs

@@ -265,18 +265,19 @@ fn eval(model: &Model) -> Output {
         return out;
     }
 
-    let mut eval = tvix_eval::Evaluation::new_pure();
-    let source = eval.source_map();
+    let mut eval_builder = tvix_eval::Evaluation::builder_pure();
+    let source = eval_builder.source_map().clone();
 
     let result = {
         let mut compiler_observer = DisassemblingObserver::new(source.clone(), &mut out.bytecode);
-        eval.compiler_observer = Some(&mut compiler_observer);
+        eval_builder.set_compiler_observer(Some(&mut compiler_observer));
 
         let mut runtime_observer = TracingObserver::new(&mut out.trace);
         if model.trace {
-            eval.runtime_observer = Some(&mut runtime_observer);
+            eval_builder.set_runtime_observer(Some(&mut runtime_observer));
         }
 
+        let eval = eval_builder.build();
         eval.evaluate(&model.code, Some("/nixbolt".into()))
     };