Combine Interoperability: first iteration#776

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andersio wants to merge 6 commits intomasterfrom

anders/combine-interop

Commits on Apr 19, 2020

Convert any Signal, Producer or Property to a Combine publisher via `publisher()`.
andersio
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Convert Combine publishers to producer via `producer()`. A few utilities.
andersio
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Update the availability annotations.
andersio
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Commits on Apr 26, 2020

Remove `Cancellable` conformances.
andersio
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Commits on Apr 27, 2020