Proposal to introduce SubSystems

[davesmith00000]

This is just a placeholder so that I don't forget.

Note that this isn't quite the same as an Indigo subsystem, although I think you could build something like that on top of this.

It is sometimes desirable to have a long running stream that is outside of the main Tyrian stream, and that has a different use-case to a Sub.

An example of this might be that the Tyrian / Indigo bridge could be rewritten to use an fs2 stream that takes in the usual Msg's and pipes the results back into the main runtime without needing to have a lump of mutable state lying around making the place untidy.

This maintains the decoupling that we aim for in the TEA pattern, but allows long lived processes and communication bridges that might consume and produce messages at uneven rates.