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/src/de_tests.rs

@@ -204,7 +204,7 @@ fn deserialize_enum_all() {
 fn deserialize_with_config() {
     let result: String = from_str_with_config("builtins.testWithConfig", |eval| {
         // Add a literal string builtin that just returns `"ok"`.
-        eval.src_builtins.push(("testWithConfig", "\"ok\""));
+        eval.add_src_builtin("testWithConfig", "\"ok\"")
     })
     .expect("should deserialize");
 
@@ -237,7 +237,7 @@ fn deserialize_with_extra_builtin() {
     let code = "builtins.prependHello \"world\"";
 
     let result: String = from_str_with_config(code, |eval| {
-        eval.builtins.append(&mut test_builtins::builtins());
+        eval.add_builtins(test_builtins::builtins())
     })
     .expect("should deserialize");