Batch updates do not need any operator

[groue]

It was ill-advised to define an <- operator for batch updates, a feature that is only known and used by a few users.

Lesson learned: Swift operators must be obvious and undisputed, or they just become a nuisance which conflict with other librairies.

Batch updates are now based on the set(to:) method:

// UPDATE player SET score = 0
try Player.updateAll(db, Column("score").set(to: 0))