Dump requests

[groue]

This PR brings the dump family of methods, suited for use in the debugger, or testing:

// Prints the full database content
try dbQueue.dumpContent()

// Prints the content of provided tables
try dbQueue.dumpTables(["player", "team"])

// Prints the content of a request
try dbQueue.dumpRequest(Player.all())

// Prints the content of SQL queries
try dbQueue.dumpSQL("SELECT * FROM player")

// Same printing methods exist on Database:
try dbQueue.read { db in
    try db.dumpContent()
    try db.dumpTables(["player", "team"])
    try db.dumpRequest(Player.all())
    try db.dumpSQL("SELECT * FROM player")
}

Several output formats are built-in, inspired by SQLite command-line tool:

// Prints one database row per line.
// The `.debug` format is suited for debugging, but not for testing.
// It may evolve over time in order to further help developers.
try dbQueue.dumpRequest(Player.all())
try dbQueue.dumpRequest(Player.all(), format: .debug())

// Prints one database value per line. All blob values are interpreted as strings.
try dbQueue.dumpRequest(Player.all(), format: .line())

// Prints one database row per line. All blob values are interpreted as strings.
try dbQueue.dumpRequest(Player.all(), format: .list())

// Prints one database row per line, formatting values as SQL literals.
try dbQueue.dumpRequest(Player.all(), format: .quote())

// Who doesn't like JSON?
try dbQueue.dumpRequest(Player.all(), format: .json())

It is just so handy to be able to dump database content at will!