Rename aliased(_:) methods to forKey(_:)

[groue]

This PR renames the method that gives an alias to an SQL column from aliased(_:) to forKey(_:):

// SELECT (width * height) AS area FROM shape
let area = (Column("width") * Column("height")).forKey("area")
let request = Shape.select(area)
if let row = try Row.fetchOne(db, request) {
    let area: Int = row["area"]
}

// SELECT author.*,
//        COUNT(DISTINCT book.rowid) AS numberOfBooks,
// FROM author
// LEFT JOIN book ON book.authorId = author.id
// GROUP BY author.id
let aggregate = Author.books.count.forKey("numberOfBooks")
let request = Author.annotated(with: aggregate)
if let row = try Row.fetchOne(db, request) {
    let author = Author(row: row)
    let numberOfBooks: Int = row["numberOfBooks"]
}

The old aliased(_:) methods are still defined, but deprecated.

The goal of this PR is to increase API consistency:

• The forKey(_:) method is now the unique way to rename values, associated records, and association aggregates, when one wants to customize the decoding of codable records, or the keys used for raw Row subscript.
• The method aliased(_:) is now only used by table aliases.