[WIP] Robin Hood Hashing

[aterenin]

This PR implements Robin Hood hashing, through introducing a new probing scheme, as per #817. An initial-pass implementation for get and insert, and tests, are done.

The implementation works by adding if constexpr logic throughout to handle Robin Hood specifics as they come up:

• The big question: is this the right structure? As an alternative, I could refactor the code to write separate insert methods for Robin Hood vs. others, and dispatch according to table type.

Some more details about this question, with an example, are given at the very end of #817.

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[PointKernel]

/ok to test 1f3d64e

[PointKernel]

@aterenin Just wanted to let you know we haven't forgotten about this work! @sleeepyjack and I are discussing the new design and will align on a concrete proposal before moving forward to minimize unnecessary iterations. We'll keep you posted.

[aterenin]

Sounds good!

The current WIP implements everything needed, along with tests. I've carefully gone over all the core logic, and I think is reasonably close to ready-to-review.

The main remaining item I was planning to think about more is whether there is a more maintenance-friendly way to organize and structure the code, which is what the "big question" in the PR centers on. I also have a list of a bunch of subtleties worth mentioning, but these should wait until a review.

If discussion would help, I'd be happy to jump on a Zoom call - just let me know!