Lint `fn new() -> Self` without Default

[mcarton]

@vi just suggested to have a lint “for having fn new() -> Self, but not having Default” on IRC.
That’s probably a good idea, but it might trigger a lot, so maybe an Allow lint by default?

[Manishearth]

Not sure if it needs to be Allow, but sure :)

[llogiq]

I'd be OK with warn, too.