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/lexer.rs

@@ -8,7 +8,9 @@ use core::str::CharIndices;
 use std::convert::AsRef;
 use std::path::Path;
 
-use crate::value::Value;
+use crate::Rc;
+use crate::Value;
+
 use anyhow::{anyhow, bail, Result};
 
 #[derive(Clone)]
@@ -20,7 +22,7 @@ struct SourceInternal {
 
 #[derive(Clone)]
 pub struct Source {
-    src: std::rc::Rc<SourceInternal>,
+    src: Rc<SourceInternal>,
 }
 
 #[derive(Clone)]
@@ -108,7 +110,7 @@ impl Source {
             lines.push((s, s));
         }
         Self {
-            src: std::rc::Rc::new(SourceInternal {
+            src: Rc::new(SourceInternal {
                 file,
                 contents,
                 lines,