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/serde/examples/nixpkgs.rs

@@ -23,8 +23,8 @@ fn main() {
     }
     "#;
 
-    let result = tvix_serde::from_str_with_config::<Config, _>(code, |eval| {
-        eval.enable_impure(None);
+    let result = tvix_serde::from_str_with_config::<Config, _>(code, |eval_builder| {
+        eval_builder.enable_impure(None)
     });
 
     match result {