Introduce request derivation protocols

[groue]

This PR introduces the four protocols SelectionRequest, FilteredRequest, AggregatingRequest, and OrderedRequest, which provide basic request derivation API. QueryInterfaceRequest adopts those protocols:

let request = Player.select(...).filter(...).group(...).order(...)

This PR is a step towards the Associations API, where QueryInterfaceRequest will no longer be the only type that provides the select and filter methods:

// soon
let frenchAuthors = Book.author.filter(Column("countryCode") == "FR")
let frenchBooks: [Book] = try Book.joined(required: frenchAuthors).fetchAll(db)

Also in this PR: request.select(sql: "...") does no longer selects a SQLExpressionLiteral, but a SQLSelectionLiteral instead (a new, internal type). The new SQLExpressionLiteral will raise a fatal error when GRDB wants to know the number of selected columns, instead of letting SQLExpressionLiteral return an ill-advised 1. This fix will make handling of joined requests, where counting columns is important, more robust.