feat: Implement Collect pattern

[EvanBoyle]

Adds a <Collect> component to enable a map/reduce or fork/join pattern. Usage:

export const CollectWorkflow = createWorkflow<
  CollectWorkflowInputs,
  CollectWorkflowOutputs
>((props, render) => {
  const [NumberCollector, doubledNums] = createCollector<number>();
  const [StringCollector, doubledStrings] = createCollector<string>();

  return (
    <>
      <NumberCollector>
        {() => props.numbers.map(n => <DoubleNumber input={n} />)}
      </NumberCollector>
      <StringCollector>
        {() => props.strings.map(s => <DoubleString input={s} />)}
      </StringCollector>
      <SumNumbers numbers={doubledNums}>
        {sum => (
          <ConcatStrings strings={doubledStrings}>
            {str => render({ num: sum, str })}
          </ConcatStrings>
        )}
      </SumNumbers>
    </>
  );
});

A few notable things:

1. Due to the fact that typescript handles type inference with JSX, the types need to be known at the time to component is instantiated, and you can't use information provided via props or children at the time it is used in markup. This requires using the createCollector HOC.
2. I had to hack into workflow builder because we wrap up the children in a function by default. I exposed __rawChildren so that Collector can access the raw children directly. Things just work.

Personally, I don't think that the collector pattern with a HOC is too bad here. The nice thing is that it returns a workflow output promise that will contain it's results - so it basically just replaces having to declare an explicit workflow output which you would otherwise have to do.

[jmoseley]

After discussion, we concluded that this pattern is not quite right.