perf: speeding up poseidon-permutation

[redshiftzero]

This PR contains some performance fixes in the implementation of poseidon-permutation. Below are some numbers based on benchmarking and profiling. Related to penumbra-zone/penumbra#1123.

From the paper the optimized version of Poseidon reduces the number of multiplications in the partial rounds.

The approximate "5x" number cited in the paper came from a Poseidon instance with t=24, R_F = 8, R_P = 42, so the number of multiplications per permutation was:
t^2 (R_F + R_P) = 24^2 * (8 + 42) = 28,800 multiplications
After the optimization, the number of multiplications in the partial rounds goes from t^2 to 2*t, so after:
t^2 * R_F + 2 * t R_P = 24^2 * 8 + 2 * 24 * 42 = 6,624 multiplications
Which is about a 4.3x reduction in multiplications in the permutation. There are big gains here because the instance is very high t, very high R_P (partial round).

For us testing with the highest width hash we use, 4:1, we have t=5, R_F = 8, R_P = 31 so for us the expected reduction in the number of multiplications:
Before: t^2 (R_F + R_P) = 5^2 * (8 + 31) = 975 multiplications
After: t^2 * R_F + 2 * t R_P = 5^2 * 8 + 5 * 2 * 31 = 510 multiplications
Which is about a 1.9x reduction in multiplications.

Comparing the empirical performance for the 4:1 parameter set derived using poseidon-paramgen:
ark-sponge: ~39.5us/hash
unoptimized poseidon-permutation (this repo): ~36.0us/hash
optimized poseidon-permutation (this repo): ~26.8us/hash

At this point from profiling the computation for the optimized permutation, we're dominated by applying the Sboxes, which take about 60% of the computation of the permutation. The partial round multiplication is down to 26%, so if further improvements are made to that area of the code we'll get some additional speedups.