Add the corresponding iterator trait to size(::Number) = ()

[mschauer]

This makes numbers well behaved with the product iterator and with collect: For example now

size(collect(1)) == size(1)

and after merging #16437 thanks to #15431

size(A[1:3,1,1:3]) == size(Base.product(1:3,1,1:3))

for an array A.

[tkelman]

There's a bit of a desire to make numbers not iterable in which case this is not meaningful, ref #7903

[mschauer]

I know, I did not want to touch that subject, it is just the right thing for the current situation.

[mschauer]

PS: I think now that this definition takes a bit of the pressure away to proceed with #7903.

[stevengj]

Regardless of whether numbers are iterable, they have to work with broadcast so that you can do things like broadcast(atan2, y, 0.3), so they have to have a size.

In which case it makes sense for them to have the corresponding trait?

(And, in fact, the broadcast case makes me want to re-think the whole "numbers should not be iterable" argument.)

[mschauer]

@stevengj Yes, exactly. I just wrote a similar statement over there in 7903.