update Sign API

[lighter-zz]

the Sign algorithm now has three APIs:

• sign arbitrary bytes (new): in this setup, the message is uniquely hashed to a set of elements
• sign canonical bytes (previously sign bytes): in this setup, the message must already be in the right shape: serialized bytes from canonical form of the field elements. otherwise the sign will fail. this is for backwords compatibility; it needs to be called with extreme care, and should be deprecated
• sign field elements: the message field elements be in the right shape. the sign will fail if the field elements is not in the right form