Type inference of selected columns

[groue]

This pull request allows to omit the as: parameter of the select(_:as:) request derivation method. It can be convenient.

For example:

extension QueryInterfaceRequest where RowDecoder == Player {
    static func maximumScore(): QueryInterfaceRequest<Int?> {
        select(max(Column("score"))) // previously we needed `, as: Int.self`
    }
}

let maxScore: Int? = try dbQueue.read { db in
    try Player.all().maximumScore().fetchOne(db)
}

This also applies to RxGRDB and GRDBCombine:

extension PlayerDatabase {
    func maximumScorePublisher() -> AnyPublisher<Int?, Error> {
        ValueObservation
            .tracking(value: Player.select(max(Column("score"))).fetchOne) // previously we needed `, as: Int.self`
            .publisher(in: dbReader)
            .eraseToAnyPublisher()
    }
}