refactor(tvix/eval): pass a VM reference to builtins

This makes it possible for builtins to force values on their own,
without the VM having to apply a strictness mask to the arguments
first.

Change-Id: Ib49a94e56ca2a8d515c39647381ab55a727766e3
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6411
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

tvix/eval/src/vm.rs

@@ -418,7 +418,7 @@ impl VM {
 
                         Value::Builtin(builtin) => {
                             let arg = self.pop();
-                            let result = fallible!(self, builtin.apply(arg));
+                            let result = fallible!(self, builtin.apply(self, arg));
                             self.push(result);
                         }
                         _ => return Err(self.error(ErrorKind::NotCallable)),