Make Engine reentrant to prepare for parallel execution.

examples/reuse_scope.rs

@@ -1,7 +1,7 @@
 use rhai::{Engine, EvalAltResult, Scope, INT};
 
 fn main() -> Result<(), EvalAltResult> {
-    let mut engine = Engine::new();
+    let engine = Engine::new();
     let mut scope = Scope::new();
 
     engine.eval_with_scope::<()>(&mut scope, "let x = 4 + 5")?;