Add constants.

tests/constants.rs

@@ -0,0 +1,16 @@
+use rhai::{Engine, EvalAltResult};
+
+#[test]
+fn test_constant() -> Result<(), EvalAltResult> {
+    let mut engine = Engine::new();
+
+    assert_eq!(engine.eval::<i64>("const x = 123; x")?, 123);
+
+    match engine.eval::<i64>("const x = 123; x = 42; x") {
+        Err(EvalAltResult::ErrorAssignmentToConstant(var, _)) if var == "x" => (),
+        Err(err) => return Err(err),
+        Ok(_) => panic!("expecting compilation error"),
+    }
+
+    Ok(())
+}