Enhance SQLLiteral and SQL interpolation

[groue]

This pull requests helps injecting raw SQL snippets in the query interface.

The context of this pull request is that GRDB does not provide Swift APIs for all SQLite expressions. There is no built-in support for CAST, CASE, STRFTIME, etc. When one wants to use those constructs, one has to use raw SQL at some point, or submit a pull request which extends the query interface ;-).

One can always write full SQL queries, but those are not quite easy to compose, and their elements can't be reused:

// How to reuse the `date(createdAt)` snippet?
// How to reuse this request and refine it?
let day = "2020-01-23"
let request: SQLRequest<Player> = "SELECT * FROM player WHERE DATE(createdAt) = \(day)"

To solve this, we enhance SQL interpolation and the SQLLiteral type:

let day = "2020-01-23"
let createdAt = Column("createdAt")
let creationDay = SQLLiteral("DATE(\(createdAt))").sqlExpression

// SELECT * FROM "player" WHERE DATE("createdAt") = '2020-01-23'
let request = Player.filter(creationDay == day)

You can wrap this DATE function in a Swift function which accepts any value accepted by the database as a date (Date, String, etc.):

func date(_ value: SQLExpressible) -> SQLExpression {
    SQLLiteral("DATE(\(lhs.sqlExpression))").sqlExpression
}

let day = "2020-01-23"
let createdAt = Column("createdAt")

// SELECT * FROM "player" WHERE DATE("createdAt") = '2020-01-23'
let request = Player.filter(date(createdAt) == day)

If you follow the recommended practices for Record types, this can give:

extension Player {
    enum Columns {
        static let createdAt = Column(CodingKeys.createdAt)
    }
}

extension DerivableRequest where RowDecoder == Player {
    func filter(creationDay: DateComponents) -> Self {
        let dbDateComponents = DatabaseDateComponents(creationDay, format: .YMD)
        return filter(date(Player.Columns.createdAt) == dbDateComponents)
    }
}

// SELECT * FROM "player" WHERE DATE("createdAt") = '2020-01-23'
let day = DateComponents(year: 2020, month: 1, day: 23)
let request = Player.all().filter(creationDay: day)