message: Remove sled_storage.rs ResultSet struct, simplify iterator codes
diff: use bincode;
  use nom_sql::CreateTableStatement;
  -use sled::{self, Db, Iter};
  +use sled::{self, Db};
  use crate::executor::Row;
  use crate::storage::Store;
  -struct ResultSet {
  - iter: Iter,
  -}
  -
  -impl Iterator for ResultSet {
  - type Item = Row;
  -
  - fn next(&mut self) -> Option<Row> {
  - self.iter
  - .next()
  - .and_then(|result| result.ok())
  - .map(|(_, value)| bincode::deserialize(&value).expect("Stop iterate"))
  - }
  -}
  -
  pub struct SledStorage {
  tree: Db,
  }
  @@ -66,8 +51,11 @@ impl Store for SledStorage {
  let k = format!("data/{}/", table_name);
  let k = k.as_bytes();
  - let iter = self.tree.scan_prefix(k);
  - let result_set = ResultSet { iter };
  + let result_set = self
  + .tree
  + .scan_prefix(k)
  + .map(|result| result.expect("should be unwrapped"))
  + .map(|(_, value)| bincode::deserialize(&value).expect("Stop iterate"));
  Ok(Box::new(result_set))
  }
  