Simplify call_fn API, no need to pass &mut references.

tests/engine.rs

@@ -4,9 +4,15 @@ use rhai::{Engine, EvalAltResult};
 fn test_engine_call_fn() -> Result<(), EvalAltResult> {
     let mut engine = Engine::new();
 
-    let ast = engine.compile("fn hello(x, y) { x.len() + y }")?;
+    let ast = engine.compile(
+        r"
+            fn hello(x, y) {
+                x.len() + y
+            }
+                ",
+    )?;
 
-    let result: i64 = engine.call_fn("hello", &ast, (&mut String::from("abc"), &mut 123_i64))?;
+    let result: i64 = engine.call_fn("hello", &ast, (String::from("abc"), 123_i64))?;
 
     assert_eq!(result, 126);