add SortAs method, sort a items slice by another keys slice

[alingse]

i have many case look like this

func FetchData() {
    var itemIDs = []int{3, 4, 1, 2}
    var items = db.Execute("select * from table where id in ?",  itemIDs).FindItems()
    return items
}

the itemIDs was not in order given by another table.
but the db return the items always by the default order by id，which was not expected as the index of itemIDs

we need to build a map[id]item and sometimes map[id][]item and then join it.

maybe we can use the exists data to do a for loop to solve the problem,but

i think SortAs is still a useful function.