arc feature to enable using Engine and other data structures from multiple threads (#142)

* `arc` feature to make engine usable from multiple threads.

`arc` is turned on by default. When enabled, std::sync::Arc
will be used instead of std::rc::Rc. The former makes regorus
types like Engine, Value, ast nodes etc Send, allowing for
usability from multiple threads.
Arc would add a performance overhead though since the reference
counting will now become atomic.

Signed-off-by: Anand Krishnamoorthi <anakrish@microsoft.com>

* Make engine and related types Debug

Signed-off-by: Anand Krishnamoorthi <anakrish@microsoft.com>

* Input, Data as json. Evaluate bool queries.

Signed-off-by: Anand Krishnamoorthi <anakrish@microsoft.com>

---------

Signed-off-by: Anand Krishnamoorthi <anakrish@microsoft.com>

tests/arc.rs

@@ -0,0 +1,33 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT License.
+
+use lazy_static::lazy_static;
+use std::sync::Mutex;
+
+use regorus::*;
+
+// Ensure that types can be s
+lazy_static! {
+    static ref VALUE: Value = Value::Null;
+    static ref ENGINE: Mutex<Engine> = Mutex::new(Engine::new());
+//    static ref ENGINE: Engine = Engine::new();
+}
+
+#[test]
+fn shared_engine() -> anyhow::Result<()> {
+    let e_guard = ENGINE.lock();
+    let mut engine = e_guard.expect("failed to lock engine");
+
+    engine.add_policy(
+        "hello.rego".to_string(),
+        r#"
+package test
+allow = true
+"#
+        .to_string(),
+    )?;
+
+    let results = engine.eval_query("data.test.allow".to_string(), false)?;
+    assert_eq!(results.result[0].expressions[0].value, Value::from(true));
+    Ok(())
+}