Add an MPMC mechanism for connecting multiple input and output flows

Through a combination of a `multicaster>`, and the `merge` and `share` operators, one is able to create a dynamic multi-producer, multi-consumer API for observables that can connect or disconnect at any time on either end. (The only limitation is that this can have at most eight producers, see #2012.)

However, this is very inconvenient to set up and get right for something that I would consider a basic building block in a very decoupled system where actors can produce messages and others can choose to consume them.

I think this would be much better expressed through a mechanism that is tailor-made for this, e.g., an MPMC buffer resource with an API similar to the existing SPSC buffer resource.