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>

src/value.rs

@@ -15,7 +15,7 @@ use serde::de::{self, Deserializer, MapAccess, SeqAccess, Visitor};
 use serde::ser::{SerializeMap, Serializer};
 use serde::{Deserialize, Serialize};
 
-pub type Rc<T> = compact_rc::Rc16<T>;
+use crate::Rc;
 
 /// A value in a Rego document.
 ///