refactor backend module

src/backend/iter.rs

@@ -0,0 +1,21 @@
+use super::{Backend, Backends};
+
+#[derive(Debug)]
+pub(crate) struct BackendIter {
+    pub(crate) next: Option<Backends>,
+}
+
+impl Iterator for BackendIter {
+    type Item = Box<dyn Backend>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        match &self.next {
+            None => None,
+            Some(b) => {
+                let result = b.get_backend();
+                self.next = b.next();
+                Some(result)
+            }
+        }
+    }
+}