Code refactor, bug fixes, code docs.

tests/engine.rs

@@ -0,0 +1,14 @@
+use rhai::{Engine, EvalAltResult};
+
+#[test]
+fn test_engine_call_fn() -> Result<(), EvalAltResult> {
+    let mut engine = Engine::new();
+
+    let ast = Engine::compile("fn hello(x, y) { x.len() + y }")?;
+
+    let result: i64 = engine.call_fn("hello", ast, (&mut String::from("abc"), &mut 123_i64))?;
+
+    assert_eq!(result, 126);
+
+    Ok(())
+}