Make all public API's return Box<EvalAltResult> to reduce footprint.

tests/arrays.rs

@@ -2,7 +2,7 @@
 use rhai::{Array, Engine, EvalAltResult, RegisterFn, INT};
 
 #[test]
-fn test_arrays() -> Result<(), EvalAltResult> {
+fn test_arrays() -> Result<(), Box<EvalAltResult>> {
     let engine = Engine::new();
 
     assert_eq!(engine.eval::<INT>("let x = [1, 2, 3]; x[1]")?, 2);
@@ -58,7 +58,7 @@ fn test_arrays() -> Result<(), EvalAltResult> {
 
 #[test]
 #[cfg(not(feature = "no_object"))]
-fn test_array_with_structs() -> Result<(), EvalAltResult> {
+fn test_array_with_structs() -> Result<(), Box<EvalAltResult>> {
     #[derive(Clone)]
     struct TestStruct {
         x: INT,