Further storage iterator refactoring (#13445)

* Remove `Backend::apply_to_key_values_while`

* Add `IterArgs::start_at_exclusive`

* Use `start_at_exclusive` in functions which used `Backend::apply_to_key_values_while`

* Remove `Backend::apply_to_keys_while`

* Remove `for_keys_with_prefix`, `for_key_values_with_prefix` and `for_child_keys_with_prefix`

* Remove unnecessary `to_vec` calls

* Fix unused method warning in no_std

* Remove unnecessary import

* Also check proof sizes in the test

* Iterate over both keys and values in `prove_range_read_with_size` and add a test