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/eval/benches/eval.rs

@@ -2,7 +2,7 @@ use criterion::{black_box, criterion_group, criterion_main, Criterion};
 use itertools::Itertools;
 
 fn interpret(code: &str) {
-    tvix_eval::Evaluation::default().evaluate(code, None);
+    tvix_eval::Evaluation::new_pure().evaluate(code, None);
 }
 
 fn eval_literals(c: &mut Criterion) {