WIP: Allow storing xnd in plasma

[saulshanabrook]

Storing xnd objects in the Plasma store will allow us to share them between multiple processes on a machine, without copying. This pull request creates a C wrapper around plasma (libplasma), creates a xndp object in libxnd that stores a reference to a plasma object and a type, and exposes a Python API to create xnd objects stored in plasma.

TODO

libplasma

• [ ] expose plasma client
  • [ ] create
  • [ ] get
  • [ ] create
• [ ] figure out how best to compile
• [ ] expose from/to binary for object IDs

libxnd:

• [ ] add init function that creates plasma object
• [ ] add get function that retrieves object from plasma and creates xnd object
• [ ] compile xndp only when arrow is included
• [ ] rename xndp to pxnd

Python

• [ ] create xndp object

Closes #4

[wesm]

Would you like to contribute the C API wrapper to Apache Arrow? I think we'd like to grow an official C API cc @robertnishihara @pcmoritz

[saulshanabrook]

@wesm Yeah, I am happy to develop that in Arrow instead of here.

[robertnishihara]

Seems like a good idea to me :)

[saulshanabrook]

I am moving this to it's own repo for now https://github.com/plures/pxnd. It's still very much in flux/an experiment, but once it's more stable I am happy to move out the libplasma part to Arrow.