56 or 57-byte encoding for Scalars?

[conradoplg]

As I mentioned in #23 I wanted to make Scalar::to/from_bytes also take/return 57-byte arrays to make it easier to work with EdDSA since RFC 8032 encodes scalars as 57-byte arrays.

I gave it a shot but that makes the X448 implementation akward because RFC 7748 encodes scalar as 56-byte arrays :smiling_face_with_tear:

This is not a huge deal, we can leave this as-is and that should be good enough, but here are some possible ideas:

• Leave as is (makes Ed448 awkward, requiring removing a byte when encoding)
• Change everything to 57-byte arrays (makes X448 awkard, requring adding a zero byte when decoding)
• Add a new method to encode as 57-byte array (to_canonical_bytes()? though to_bytes() is also canonical...)

[kevaundray]

Hey, thanks for checking this out :)

I'd opt for either leaving as is, or adding a new method, so it doesn't break anything, maybe something like to_bytes_rfc_8032 ?

Will delegate that decision to you!