fix(tvix/eval): ensure callable is forced when using call_with

When passing multiple arguments, every intermediate callable needs to
be forced as this is expected by the VM's call_value function.

Also adds a debug assertion for this which makes it easier to spot
exactly what went wrong.

Change-Id: I3aa519cb6cdaab713bd18282bef901c4cd77c535
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7312
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/value/thunk.rs

@@ -133,6 +133,10 @@ impl Thunk {
         }
     }
 
+    pub fn is_evaluated(&self) -> bool {
+        matches!(*self.0.borrow(), ThunkRepr::Evaluated(_))
+    }
+
     /// Returns a reference to the inner evaluated value of a thunk.
     /// It is an error to call this on a thunk that has not been
     /// forced, or is not otherwise known to be fully evaluated.