Enhancements to logical operators

[groue]

The joined(operator:) method lets you join several conditions with the AND or OR SQL operators:

// SELECT * FROM players
// WHERE (registered
//        AND (score >= 1000)
//        AND (name IS NOT NULL))
let conditions = [
    Column("registered"),
    Column("score") >= 1000,
    Column("name") != nil]
Player.filter(conditions.joined(operator: .and))

When the sequence is empty, joined(operator: .and) returns true, and joined(operator: .or) returns false:

// SELECT * FROM players WHERE 1
Player.filter([].joined(operator: .and))

// SELECT * FROM players WHERE 0
Player.filter([].joined(operator: .or))