support slice of custom types in expandSliceArgs

[Songmu]

Support slice of custom types like []UserID(type UserID int64).

In this change, The expandSliceArgs expand if the slice element implements driver.Valuer or is of a named primitive type.

If the slice itself is of a certain type and implements driver.Valuer, expandSliceArgs does not expand the slice. So it keeps backward compatibility.

Typing as follows is reassuring in terms of type safety, and we define types in that way on our production.

In that case, this change is beneficial. What do you think?

type UserID int64

type User struct {
    ID UserID `db:"id"`
}