Association Aggregates

[groue]

This pull request builds on top of #422 and brings Association Aggregates, inspired by Django ORM.

For example:

// Authors with the number of books they wrote
let request = Author.annotated(with: Author.books.count)

// Authors with their minimum and maximum book year
let request = Author.annotated(with:
    Author.books.min(Column("year")),
    Author.books.max(Column("year")))

// Authors who did not write any book
let request = Author.having(Author.books.isEmpty)

// Authors who wrote more books that they did paintings
let request = Author.having(Author.books.count > Author.paintings.count)