2 tiny modification on snapshot (#8601)

* Some tiny modifications.
1. fix some typo in the comment.
2. sort the order of methods in 'impl state::Backend for StateDB`

* Remove the clone of code_cache, as it has been done in clone_basic.

* remove From::from. It seems not necessary.

* change mode: remove rust files' executable mode.

* 2 tiny modifications on snapshot.