vine: Compose remote function calls

[btovar]

For #4043.

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[dthain]

I have no objection, but I also haven't deeply understood the problem yet. Perhaps ask @tphung3 who has been thinking about functions and context?

[tphung3]

I just tested the new test code and the serverless_composed test runs with the current master (without the proposed changes). I believe this is because cloudpickle inspects each function's possible call stack and also pickles the functions that the original function depends on. So composed functions should not be a problem. @btovar Can you confirm this on your end? If true then perhaps the problem in #4011 is something else?

[btovar]

Thanks @tphung3! I'm trying to figure out what not worked before.