[DONT' MERGE] simple babybear arithmetic examples

[kunxian-xia]

Introduction

primitive	test cmd
base/ext field arithmetic	cargo test --release --lib test_ext_mul -- --nocapture
fri_fold	cargo test --release --lib basefold::test::test_fri_fold -- --nocapture
bit reverse	cargo test --release --lib basefold::test::test_bit_reverse -- --nocapture
fix var in sumcheck	cargo test --release --package multilinear_extensions --lib test_fix_var
matrix multiply by vector (derive codeword)	cargo test --release --package mpcs --lib test_matrix_multiply_vector

• infer product witness
• infer logup witness
• ntt
• poseidon2

@maurice-mao The cmd to get these examples are

cargo test --release --lib test_ext_mul -- --nocapture