feat(tvix/eval): add Evaluation::strict to toggle top-level deepseq

This makes it possible for callers to control whether they can receive
partially evaluated values from an evaluation or not.

We're actually flipping the default behaviour to non-strict top-level
evaluation, which means that callers have to set `strict = true` on
the Evaluation to get the previous behaviour.

Change-Id: Ic048e9ba09c88866d4c3177d5fa07db11c4eb20e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8325
Autosubmit: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/tests/mod.rs

@@ -52,6 +52,7 @@ fn eval_test(code_path: &str, expect_success: bool) {
     }
 
     let mut eval = crate::Evaluation::new_impure(&code, Some(code_path.into()));
+    eval.strict = true;
     eval.builtins.extend(mock_builtins::builtins());
 
     let result = eval.evaluate();
@@ -100,6 +101,7 @@ fn identity(code_path: &str) {
     let code = std::fs::read_to_string(code_path).expect("should be able to read test code");
 
     let mut eval = crate::Evaluation::new(&code, None);
+    eval.strict = true;
     eval.io_handle = Box::new(crate::StdIO);
 
     let result = eval.evaluate();