Implement variable resolver.

doc/src/safety/progress.md

@@ -7,7 +7,8 @@ It is impossible to know when, or even whether, a script run will end
 (a.k.a. the [Halting Problem](http://en.wikipedia.org/wiki/Halting_problem)).
 
 When dealing with third-party untrusted scripts that may be malicious, to track evaluation progress and
-to force-terminate a script prematurely (for any reason), provide a closure to the `Engine::on_progress` method:
+to force-terminate a script prematurely (for any reason), provide a closure to the [`Engine`] via
+the `Engine::on_progress` method:
 
 ```rust
 let mut engine = Engine::new();