Proposal for serialization support

[Nashtare]

We currently support two types of encoding:

• CanonicalSerialize / CanonicalDeserialize -> those are easily derived as all our wrapper types rely on arkworks traits that do implement those;
• custom to_bytes / from_bytes inherited from the old codebase, which are simply calling CanonicalSerialize / CanonicalDeserialize methods internally.

This issue concerns two points related to serialization:

• addressing the code duplication induced by all the custom to_bytes / from_bytes methods through new traits, like Encode/ToBytes and Decode/FromBytes (names are arbitrary and not carved in marble), which would be supertraits of CanonicalSerialize and CanonicalDeserialize respectively.

• We may want to additionally support serde behind some feature flag for all our public types. This could actually remove the need for a custom serialization mechanism, or be complementary to it (for users who would like to rely on as few extra dependencies as possible and stick to the custom Encode/Decode).

Any thoughts?