draft: poseidon2 (plonky3) integration in benchtop

[rphmeier]

I'm opening this PR as a placeholder, but it was interesting to integrate Poseidon2 with NOMT.

This creates a custom Poseidon2 hasher for nodes and uses the Legendre Symbol (Residue/Non-Residue)
rather than the MSB of the hash to indicate leaf/internal nodes.

This is not optimized at all but still comfortably achieves 40k+ TPS on a benchmark with 2^25 items.

Hashes are of the form [KoalaBear; 8]. I convert blake3 value hashes into this type by taking each 32-bit word modulo the KoalaBear prime.

Key-paths are treated specially. Because NOMT internals require that the key-path is encoded as a bit-string, I instead limited the length of the key-path to 240 bits and treat them as 8 densely packed 30-bit limbs. i.e. a KeyPath is a [KoalaBear; 8] where none of the field elements (when treated as a 32-bit number) use more than 30 bits, and which are densely packed into the [u8; 256]

[rphmeier]

• draft: poseidon2 (plonky3) integration in benchtop #782 👈 (View in Graphite)
• refactor: define hasher traits in nomt-core hasher module and feature-gate #787 : 1 other dependent PR (#788 )
• refactor: put node hasher in charge of labeling nodes and determining kinds #781
• master

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