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

tests/unit.rs

@@ -1,21 +1,21 @@
 use rhai::{Engine, EvalAltResult};
 
 #[test]
-fn test_unit() -> Result<(), EvalAltResult> {
+fn test_unit() -> Result<(), Box<EvalAltResult>> {
     let engine = Engine::new();
     engine.eval::<()>("let x = (); x")?;
     Ok(())
 }
 
 #[test]
-fn test_unit_eq() -> Result<(), EvalAltResult> {
+fn test_unit_eq() -> Result<(), Box<EvalAltResult>> {
     let engine = Engine::new();
     assert_eq!(engine.eval::<bool>("let x = (); let y = (); x == y")?, true);
     Ok(())
 }
 
 #[test]
-fn test_unit_with_spaces() -> Result<(), EvalAltResult> {
+fn test_unit_with_spaces() -> Result<(), Box<EvalAltResult>> {
     let engine = Engine::new();
     engine.eval::<()>("let x = ( ); x")?;
     Ok(())