Implement import keyword (#101)

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

src/engine.rs

@@ -100,6 +100,7 @@ impl Engine {
             self.interpreter
                 .set_functions(gather_functions(&self.modules)?);
             self.interpreter.gather_rules()?;
+            self.interpreter.process_imports()?;
             self.prepared = true;
         }