Query Interface: set the selection and the fetched type in a single method call

[groue]

This PR introduces the new select(..., as:...) method:

// new
let request = Player.select(max(Column("score")), as: Int.self)

This new method does not bring any new functionality. It is 100% equivalent to the already available select(...).asRequest(of:...):

// old way to do it
let request = Player
    .select(max(Column("score")))
    .asRequest(of: Int.self)

The goal of this PR is to help building apis that return requests (instead of returning fetched values). For example:

struct Player: Codable, FetchableRecord, PersistableRecord {
    var uuid: UUID
    var name: String
    var score: Int
    
    private enum CodingKeys: String, CodingKey, ColumnExpression {
        case uuid, name, score
    }
    
    static var maximumScore: QueryInterfaceRequest<Int> {
        // new
        return select(max(CodingKeys.score)), as: Int.self)
    }
    
    static func uuid(forName name: String) -> QueryInterfaceRequest<UUID> {
        // new
        return filter(CodingKeys.name == name)
            .select(CodingKeys.uuid, as: UUID.self)
    }
}

// Fetch
dbQueue.read { db in
    let maximumScore: Int? = try Player.maximumScore.fetchOne(db)
    let uuid: UUID? = try Player.uuid(forName: "Arthur").fetchOne(db)
}

// Observe with RxGRDB
Player.maximumScore.rx
    .fetchOne(in: dbQueue)
    .subscribe(onNext: { maximumScore: Int? in
        print("maximum score has changed")
    })