Reconsider all of Skelecortex.Plumbing

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Skelecortex.Plumbing was a really fast brain to code dump of an intended simplification (in .NET Standard) of some concepts from TPL.

So far, it doesn't feel simple enough - it needs a lot more thought.

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Typically, in the future, something like this will remain in its own branch until it's better thought out, but I intend to resolve one soon and it was part of my local efforts when I decided to push it all to github.

I consider this somewhat a dependency to future efforts as I'd like to make use of the running concept of "pipelines" (as well as synchronization) in other libraries' asynchronous needs. I need to get this library ironed out for light-weight practical use before adding a whole lot more.

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Removed from master and put into its own branch: Plumbing-Rethought