Direct Solver: Multilevel Proxy-based Solver

[alexfikl]

Attempting to break up the direct solver MR into smaller, more reviewable pieces. As a rundown

• [Done] Add proxy generation (already in linalg.proxy).
• [Done] Add single-level skeletonization (Direct Solver: Skeletonization #30).
• [WIP] Add multi-level skeletonization (Direct Solver: Recursive Skeletonization #164).
• [WIP] Add the actual multi-level direct solver (Direct Solver: Multilevel Proxy-based Solver #174).

The code for this is mostly in the deprecated https://gitlab.tiker.net/inducer/pytential/-/merge_requests/137.

[alexfikl]

This also contains #164 at the moment and is still quite WIP. For one, only implemented a forward matvec, not the actual solver 😢

[alexfikl]

This now implements both a matvec and a direct solver with recursive skeletonization. Both seem to work pretty close to design parameters. Still need to stress test it more though!