Simplified function registration to not require explicit coercion step. Simplified eval to take &str instead of String

examples/hello.rs

@@ -4,7 +4,7 @@ use rhai::Engine;
 fn main() {
     let mut engine = Engine::new();
 
-    if let Ok(result) = engine.eval("40 + 2".to_string()).unwrap().downcast::<i32>() {
+    if let Ok(result) = engine.eval("40 + 2").unwrap().downcast::<i32>() {
         println!("Answer: {}", *result);  // prints 42
     }
 }