Add argless `#update` and `#delete` instance query shortcuts

[vladfaust]

repo.exec(user.update(updated_at: Time.now))

Doesn't work now, as Model#update requires Changeset argument.

[vladfaust]

I think that it would be better to add argless update and delete shortcuts:

user.update == User.where(id: user.id!).update

The first example would be changed to:

repo.exec(user.update.set(updated_at: Time.now))