fix: check canonical Fp elements

[Rjected]

This adds the negative test vectors that surfaced some issues in the EIP-2537 implementation. The issue was that the blst_fp_from_bendian method did not check that the blst_fp or blst_fp2 is canonical. There does not seem to be any method in blst for checking this, so instead we convert to big-endian again, checking that the input equals the re-encoded field element.

Finding a better way to do this would be highly appreciated.

This is encapsulated into check_canonical_fp and check_canonical_fp2.

After this change, the positive and negative test vectors from the EIP pass.