Expose 4 ways to create a field element from arbitrary bytes:
- `from_bytes_be`/`from_bytes_le`: take exactly 32 bytes and interpret
  them as a big/little endian number of 256 bits and then build a field
  element from it, modulo STARK prime (in whatever representation
  `Stark252PrimeField` uses, currently Montgomery);
- `from_bytes_be_slice`/`from_bytes_le_slice`: take any number of bytes
  and interpret them as a big/little endian number modulo STARK prime by
  decomposing into 256 bits integers and multiplying by the appropriate
  factors.