Add an `empty` method to each collection

[julienrf]

See scala/collection-strawman#145 (comment) and scala/scala3#2573 for motivation.

The following should be enough:

def empty: CC[A] = iterableFactory.empty[A]

Or maybe:

def empty: C = fromSpecificIterable(iterableFactory.empty[A])

[julienrf]

Note that the changes proposed in scala/scala3#2573 (comment) not only require empty but also point (in case there is no subsequent generator).

[szeiger]

Let's see what Dotty does about this and maybe implement it in 2.14.

[dwijnand]

Are there any collections that don't have an empty?

[NthPortal]

it's defined on IterableOps, so all collections have one