Reject identity in Group::serialize

[doutv]

In current RFC document 3.1, page 10:

SerializeElement(A): Maps an Element A to a canonical byte array
buf of fixed length Ne. This function raises an error if A is the
identity element of the group.

Serialize a Group identity element should raise error.

Related PR:
cfrg/draft-irtf-cfrg-frost#306
#145

[conradoplg]

Thank you for reporting this! It was good timing since the next release will be an API-breaking one.