1.x: lift into Subject

[akarnokd]

See #2458.

This PR shows a way to implement lifting into a Subject. It is possible to lift on the front and the back side. The main idea is to add behavior to a Subject while presenting it to the outside world still as a Subject.

I'm not 100% certain the value of this because one has to write operators for this to work (relying on the rx.internal.operators is not recommended). The front-side operator can be simpler since it doesn't have to deal with unsubscription and backpressure as the wrapped Subjects don't have that capability on their front anyway.

[stevegury]

I'm not convinced either by its value, do you have a example in mind?

[akarnokd]

This was @abersnaze 's long lasting dream from #2458. I thought I give an implementation to see if we want it or #2458 can be closed for good.

[abersnaze]

I wouldn't go as far as saying it was a dream of my but it has always bugged me that the type Subject<T, R> clearly has an R that can be different but there isn't a practical way to change it. If subject is never intended to be used like that then we should change it to Subject<T> in 2.0.