Switch to big-endian

[jtraglia]

This PR changes the endianness from little-endian to big-endian, as defined here:

• https://github.com/ethereum/consensus-specs/pull/3354

See also:

• https://github.com/ethereum/c-kzg-4844/pull/305

[jtraglia]

No problem 😄 your changes LGTM 👍