Worker Threads

It is absolutely trivial to use thew worker threads feature in Node with Highland. I'm not sure if this should be part of core but at the very least I think we should document it:

Example here: https://github.com/svozza/highland-worker-thread

Start by reviewing the worker-thread example at https://github.com/svozza/highland-worker-thread and the existing Highland documentation. Clarify whether the contribution should document external usage or add core support; done means the supported scope and a usable worker-threads example are documented.

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