Improve serialization of Publickey and Signature

[drskalman]

arkworks now have serialization for projective curve, so we should use it.

[drskalman]

ffbcbff derive serialization and impl ToBytes FromBytes from arkworks. Tests should be adapted to use ToBytes FromBytes. The compromise is that to bytes and from bytes do not return constant length arrays. But that was broken and hardcoded for 384 bytes.

[burdges]

Avoid To/FromBytes: arkworks-rs/algebra#390