Added foldLeft method

I want to folding for join queries or map building.

```scala
type EmployeeId = Int
type GroupId = Int

val result: Map[GroupId, Set[EmployeeId]] = 
  sql"SELECT id, group_id FROM employee".foldLeft(Map.empty[GroupId, Set[EmployeeId]].withDefaultValue(Set.empty)) {
    case (map, rs) => 
      val gId = rs.int("group_id")
      val eId = rs.int("id")
      map + (gId -> (map(gid) + eId))
}
```