Make Engine reentrant to prepare for parallel execution.

tests/bit_shift.rs

@@ -2,14 +2,14 @@ use rhai::{Engine, EvalAltResult, INT};
 
 #[test]
 fn test_left_shift() -> Result<(), EvalAltResult> {
-    let mut engine = Engine::new();
+    let engine = Engine::new();
     assert_eq!(engine.eval::<INT>("4 << 2")?, 16);
     Ok(())
 }
 
 #[test]
 fn test_right_shift() -> Result<(), EvalAltResult> {
-    let mut engine = Engine::new();
+    let engine = Engine::new();
     assert_eq!(engine.eval::<INT>("9 >> 1")?, 4);
     Ok(())
 }