Use LLVM backend Python package

[Baltoli]

This supersedes the draft changes in #2844 with a more robust solution as suggested by @tothtamas28.

The changes in this PR add the kllvm bindings as a "path" dependency to pyk, and updates the nix flake build accordingly. It also adds a trivial test case to make sure that pyk can call into the bindings correctly.

Because we point to a relative path dependency for the bindings library, we need to perform a few more steps for pyk's make install target. This issue gives some more context.

• Requires Install KLLVM python package llvm-backend#553 to go through and the submodule to be updated before this will pass CI.
• Also requires Build LLVM backend Python bindings in flake llvm-backend#554 to have passed the submodule update as well.
• Additionally, we'll need the commit hash for the LLVM backend to point to the correct branch in the flake's lock file.

[Baltoli]

python-poetry/poetry#5273 (comment) when debian packaging

[dwightguth]

it's not clear to me how these changes would integrate with #2858 . You probably should coordinate in that regard and one of these two should be merged first. Probably that one, not this one.

[dwightguth]

I think it's probably more appropriate to just add Freeman to the admin team than to explicitly hardcode his username here.

[Baltoli]

Yes, I started working on this PR before I was aware of the work ongoing to pull Pyk out into its own repo. I will be coordinating with @tothtamas28 on the best way to get these changes working under that setup..