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

examples/simple_fn.rs

@@ -8,9 +8,9 @@ fn add(x: i32, y: i32) -> i32 {
 fn main() {
     let mut engine = Engine::new();
 
-    &(add as fn(x: i32, y: i32)->i32).register(&mut engine, "add");
+    engine.register_fn("add", add);
  
-    if let Ok(result) = engine.eval("add(40, 2)".to_string()).unwrap().downcast::<i32>() {
+    if let Ok(result) = engine.eval("add(40, 2)").unwrap().downcast::<i32>() {
        println!("Answer: {}", *result);  // prints 42
     }
 }