=str #15755 #15756 correct variance annotations

- make HasNoX into pure marker traits
- make materialization type-safe
- remove casting when using EmptyPublisher
=str #15755 mandate materializer for IterableSource

* Source attach to top flow subscriber
=str #15755 implement Sources

* Implement IteratorSource
* and add tests for IterableSource and IteratorSource
* Implement ThunkSource
* Implement FutureSource
* Implement TickSource
=str #15756 Implement Sinks

* Implement FutureSink
* also, make PublisherSink an object
* Implement OnCompleteSink
* Implement ForeachSink
* Implement SubscriberSink
=str #15755 #15756 Some cleanup
=str #15755 #15756 rework Source/Sink materialization

The philosophy is that the FlowMaterializer has complete control over
how it interprets the AST, no restrictions. Therefore it only involves
one specified method: materialize() which returns a MaterializedFlow.
Within the ActorBasedFlowMaterializer we materialize Sources and Sinks
that implement the specified SimpleSource/SourceWithKey interfaces (same
for Sinks), others are not supported. These traits are extensible and
they require that an ActorBasedFlowMaterializer is passed into the
factory methods. Other materializers can of course interpret these AST
nodes differently, or they can use the actor-based facilities by
creating a suitable materializer for them to use.

This means that everything is fully extensible, but the infrastructure
we provide concretely for ourselves is built exactly for that and
nothing more. Overgeneralization would just lead nowhere.