Engine: support multiple inputs to a step

[josephjclark]

This PR enables the runtime call each step multiple times.

Each time a step is called, it passes its output to any downstream steps, and so the downstream steps are run multiple times.

It basically treats a repeated step as if it were a unique step in the workflow. So if step x is run twice, the runtime basically runs x-0 and x-1 as if there were defined on the plan. Note that actually call x-0 x, because otherwise 8 million tests break. And I think it's a bit cleaner?

Issue

Closes #698

Changes

• Removed a validation step that looks for steps with multiple inputs and throws
• Adjust the core runtime to ensure that a step can have multiple inputs AND outputs
• Update unit tests

Important Behavioural notes

These are the rules of this feature:

• Execution is in SERIES and BREADTH FIRST. If start goes to a and b, and a and b go to x, the execution order is start -> a -> b -> x -> x
• State is isolated between steps. So when x is called twice, it is with two different state objects. This is what you want, but might not be what you expect. You cannot aggregate stuff in x.
• If a leaf node runs multiple times, each output will be given a unique id
• A leaf node which returns no state is ignored
• If run ends with only a single leaf step returning state, that state will be the run output. So the output will be like { data: {} }
• If a run end with multiple leaf steps returning state, each step will report its own output (this includes one step returning multiple outputs). So the output will be like { a: { data: {}, b: { data: {}, 'b-1': { data: {} } }

[josephjclark]

Hey @mtuchi is there any chance you can run a test of this for me?

You'll need to install openfnx from this branch and run a workflow through the CLI. It should work for any workflow where you want to call the same step from multiple places

[mtuchi]

@josephjclark my initial test passed 👍🏽, i want to test important behaviors that you described in depth i will share more feedback later today