General: flow streams should be able to cascade into other streams

[eledhwen]

Big evolution of both the graph model and the compiler/runtime implementations.

• stream nodes should be able to cascade into other streams
• as a result, node types involved with the interface between flow and stream modes (namely, StreamAccumulator) are evolving to be less specific, leveraging recently introduced CurrentOut signatures
• as a result, collector nodes are evolving to be less specific, with 0-n Output results instead of 0-1

Sideshow:

• adding GitHub workflow-based deployments