Expose joining methods on both requests and associations

[groue]

This PR is one more stone added on the empowering of constrained extensions to the DerivableRequest protocol. Such extensions are, in spirit, very close to Active Record Scopes. They allow to extend the query language, and reuse code between requests and associations for filtering, ordering, etc.

Joining methods are now available for the DerivableRequest protocol. For example:

/// Extends the query language for both Book requests and associations:
extension DerivableRequest where RowDecoder == Book {
    /// Filter books whose author is from the given country
    func filter(countryCode: String) -> Self {
        // Require that books can be joined to an author from this country:
        return joining(required: Book.author.filter(Column("countryCode") == countryCode)
    }
}

// Usage

// French books
let frenchBooks: [Book] = try Book.all()
    .filter(countryCode: "FR")
    .fetchAll(db)

// All libraries that contain Italian books
let libraries: [Library] = try Library
    .having(Library.books.filter(countryCode: "IT").isEmpty == false)
    .fetchAll(db)

Extensions to DerivableRequest are designed to be easily composable. Particularly, joining methods "hidden" in such extensions can freely be composed with other uses of associations, thanks to #422:

// French books with their author
struct BookInfo: Decodable, FetchableRecord {
    var book: Book
    var author: Author
}
let request = try Book.all()
    .filter(countryCode: "FR")
    .including(required: Book.author)
let bookInfos = try BookInfo.fetchAll(db, request)

• Update the Good Practices for Designing Record Types with a much stronger focus on DerivableRequest. This is the future.