Common API for requests and associations derivation

[groue]

This PR introduces DerivableRequest, a protocol that lets you enrich the query interface API. Write one extension, derive both requests and associations:

// One extension...
extension DerivableRequest where RowDecoder == Author {
    func filter(country: String) -> Self {
        return filter(Column("country") == country)
    }
}

// Apply to a request
let frenchAuthors = Author.all().filter(country: "FR")

// Apply to an association
let frenchBooks = Book.joining(required: Book.author.filter(country: "FR"))