Vine: add library context implementation

[tphung3]

Proposed Changes

This PR adds the mechanisms needed so that FunctionCalls can pull data hosted in a library on a remote node.

Merge Checklist

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• make test Run local tests prior to pushing.
• make format Format source code to comply with lint policies. Note that some lint errors can only be resolved manually (e.g., Python)
• make lint Run lint on source code prior to pushing.
• Manual Update Update the manual to reflect user-visible changes.
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[dthain]

Thanh, what's the status on this one?

[tphung3]

This is on hold at the moment. I'm working on whether to add the abstraction of context for task groups.

[dthain]

This smells like trouble to me. Why does the worker need to know if this is a future file as opposed to the creation of a regular file?

[tphung3]

I think this belongs to the future function calls and I don't modify that code except left-indent that code block in. @JinZhou5042 I see that you added this line in #3670. Can you shed some insights please?

[tphung3]

@dthain the new change send a string as exec_mode from Python to C code, and C code converts the string into an enum to use within the C codebase.

[dthain]

RTM?

[tphung3]

Tests for special functions (lambda, dynamically executed code) are added with some fix to the PR. This PR is RTM @dthain.