Make AnonymousSubject public in v6+

[Sawtaytoes]

Feature Request

Is your feature request related to a problem? Please describe.
In a prior issue (#2002), AnonymousSubject was made public. Problem is, AnonymousSubject is no longer in the main index.js file in v6. I have to go to rxjs/internal to get to it.

Describe the solution you'd like
I'd like to see AnonymousSubject made public again in v6 and on.

Describe alternatives you've considered
Using rxjs/internal.

Additional context
In RxJS v5, I made a SignalRHubConnection observable for the C# SignalR similar to the WebSocket version which also used AnonymousSubject. I don't feel comfortable referencing things in rxjs/internal for my own observables using internal functionality.

[frankwallis]

Reference to same v5 issue: #2002

Also I have found that importing from rxjs/internal can cause big increases in the webpack bundle size for some reason.

[kwonoj]

Related with #3513 : eventually we may want to change fate of anonymous subject itself, not sure we'll expose it as public interface at the moment.