refactor(tvix/eval): move Evaluation::{default,new_pure}() again

Have a Evaluation::new() function that's used to set up the Evaluation
struct initially - which is also used by both new_pure and new_impure
internally.

It's generic over the exact type of IO, making it easier to instantiate
Evaluation with non-tvix-eval EvalIO implementations, that might not be
in a Box.

Change-Id: Ibf728da24aca59639c5b6df58d00ae98c99a63f5
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10640
Reviewed-by: raitobezarius <tvl@lahfa.xyz>
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/serde/src/de.rs

@@ -46,7 +46,7 @@ where
     F: FnOnce(&mut Evaluation<Box<dyn EvalIO>>),
 {
     // First step is to evaluate the Nix code ...
-    let mut eval = Evaluation::default();
+    let mut eval = Evaluation::new_pure();
     config(&mut eval);
 
     eval.strict = true;