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/src/tests/mod.rs

@@ -117,11 +117,8 @@ fn eval_test(code_path: PathBuf, expect_success: bool) {
 fn identity(#[files("src/tests/tvix_tests/identity-*.nix")] code_path: PathBuf) {
     let code = std::fs::read_to_string(code_path).expect("should be able to read test code");
 
-    let eval = crate::Evaluation {
-        strict: true,
-        io_handle: Box::new(crate::StdIO) as Box<dyn EvalIO>,
-        ..Default::default()
-    };
+    let mut eval = crate::Evaluation::new(Box::new(crate::StdIO) as Box<dyn EvalIO>, false);
+    eval.strict = true;
 
     let result = eval.evaluate(&code, None);
     assert!(

tvix/eval/src/tests/one_offs.rs

@@ -25,7 +25,7 @@ fn test_source_builtin() {
 
 #[test]
 fn skip_broken_bytecode() {
-    let result = Evaluation::default().evaluate(/* code = */ "x", None);
+    let result = Evaluation::new_pure().evaluate(/* code = */ "x", None);
 
     assert_eq!(result.errors.len(), 1);