Make topoRank field public

[raquo]

val topoRank being protected[airstream] prevents users from extending EventStream or Signal, which is silly. The field should probably just be public.

EDIT: Is this actually true? I think I confused something. People are extending EventStream just fine it seems.

[narma]

It's actually true, I can make custom Observable only under com.raquo.airstream package, which is not good practice.

[raquo]

@narma I think you can overcome this issue by saying override val topoRank (not override protected[airstream] val topoRank). Seems to compile in 2.13 at least.

[raquo]

This is now documented in the docs, and with the new CustomSource API extending observables shouldn't be as common.

Also, for reading topoRank, there's a static Observable.debugTopoRank(obs) method now