Possible performance improvements in keccak related hints

[entropidelic]

• value in compare_bytes_in_word_nondet:

This works fine, but it should be checked for a performance improvement.
One option is to try to convert n_bytes into usize, with failure to do so simply
making value be 0 (if it can't convert then it's either negative, which can't be in Cairo memory
or too big, which also means n_bytes > BYTES_IN_WORD). The other option is to exctract
bigint!(BYTES_INTO_WORD) into a lazy_static!

• value in compare_keccak_full_rate_in_bytes_nondet: same comment as before.
• padding in cairo_keccak_finalize
• passing bigint_values as an iterator instead of a vector in u64_array_to_bigint_vec helper function