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/tests/nix_oracle.rs

@@ -41,6 +41,7 @@ fn nix_eval(expr: &str) -> String {
 fn compare_eval(expr: &str) {
     let nix_result = nix_eval(expr);
     let mut eval = tvix_eval::Evaluation::new(expr, None);
+    eval.strict = true;
     eval.io_handle = Box::new(tvix_eval::StdIO);
 
     let tvix_result = eval