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/number.rs

@@ -3,7 +3,6 @@
 
 use core::fmt::{Debug, Formatter};
 use std::cmp::{Ord, Ordering};
-use std::rc::Rc;
 use std::str::FromStr;
 
 use anyhow::{anyhow, bail, Result};
@@ -11,6 +10,8 @@ use anyhow::{anyhow, bail, Result};
 use serde::ser::Serializer;
 use serde::Serialize;
 
+use crate::Rc;
+
 pub type BigInt = i128;
 
 type BigFloat = scientific::Scientific;