Refactor WorkflowHost as a stream transformation

[zach-klippenstein]

I am thinking about ultimately defining WorkflowHost as a stream transformation. E.g., using the new preview cold streams library (Kotlin/kotlinx.coroutines#254):

val inputs: Flow<WfInput>
val outputsAndRenderings: Flow<Pair<WfRendering, WfOutput?>> = inputs.flatMapWorkflow(myWorkflow)

It already has a stream of output events, but since we don't have a native kotlin (not to be confused with kotlin native) streams library yet, the host has to have its own lifetime and vend a hot channel. I'd like to turn that into a cold stream that only fires up the workflow universe when subscribed to, and tears it down when that subscription is disposed.

Issue #247 is a proposal for a stream of inputs. With that, WorkflowHost will have a transformer shape - stream in, stream out. So we can think of a workflow as a fancy way to write reactive operators declaratively.

Tagging this for post-v1.0 because Flow is still preview, so to do this properly we'd currently need to give the runtime direct dependencies on third-party Rx libraries which would be messy.

[zach-klippenstein]

@timdonnelly @davidapgar How would you feel about this on the Swift side?

[zach-klippenstein]

This was closed by #282.