Implement comparison operators. Formalize semantics.

Rename compare.rs to comparison.rs to match OPA documentation of builtins.

Also update loop handling to ensure that the current scope is restored after
each iteration of the loop so as to undo side effects of the iteration.

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

src/value.rs

@@ -87,8 +87,8 @@ pub enum Value {
     // Json data types. serde will automatically map json to these variants.
     Null,
     Bool(bool),
-    String(String),
     Number(Number),
+    String(String),
     Array(Rc<Vec<Value>>),
     Object(Rc<BTreeMap<Value, Value>>),