feat: or keyword (#315)

Add `or` operator to Rego languages. Available via `rego-extensions`
Cargo feature.

If the evaluated lhs value is not false, null or undefined it is returned.
Otherwise rhs is evaluated and returned.

or operator has least precedence, and is left-associative.

closes #314

src/scheduler.rs

@@ -258,6 +258,12 @@ pub fn traverse(expr: &Ref<Expr>, f: &mut dyn FnMut(&Ref<Expr>) -> Result<bool>)
             traverse(rhs, f)?;
         }
 
+        #[cfg(feature = "rego-extensions")]
+        OrExpr { lhs, rhs, .. } => {
+            traverse(lhs, f)?;
+            traverse(rhs, f)?;
+        }
+
         Membership {
             key,
             value,