Reduce witness columns in Poseidon machine by slightly increasing the constraint degree

[georgwiese]

We recently decreased the degree of constraints in the Poseidon machine by introducing more witness columns, so that we make proofs with Plonky3.

But they don't need to be degree 2, degree 3 is low enough. In fact, the maximum degree in the machines is 3 anyway.

With this PR, the number of witness columns reduces significantly for PoseidonGL and PoseidonGLMemory (67 -> 43; 85 -> 61). We also save 24 polynomial identities.

To test:

cargo run -r pil test_data/std/poseidon_gl_memory_test.asm -o output -f --prove-with plonky3-composite
cargo run -r pil test_data/std/poseidon_gl_test.asm -o output -f --prove-with plonky3-composite