Basic serde support for Scalar, G1 and G2 (Fix #57)

[akakou]

Fix CI error of #57.

This PR makes this library support structs as below to serde.

• Scalar
• G1Affine and G1Projective
• G2Affine and G2Projective

[akakou]

It related to #35, #38, #57.

[str4d]

I think this PR needs a rebase now that #64 has been merged.

Also, as a heads up: this PR is unlikely to be part of bls12_381 0.5.0 (which will be cut later this week) due to time constraints. But since the (implicit) serde feature flag is completely optional, we could include it in a subsequent point release.

[akakou]

Thank you for your reply!
I rebased it, so could you check this PR?

[ma2bd]

Any time estimate for the landing of this PR? Serde is useful not only for serialization but also for the generation of correct message hashers (see https://crates.io/crates/bcs).

[akakou]

@str4d

I solved the conflicts.
Could you check and merge this PR?
(I hope the PR be merged before when the next conflict occurs.)

[ebfull]

We introduced serde support in the pasta_curves crate here: zcash/pasta_curves#48

The implementation uses serde's is_human_readable to check whether the format it's encoding into is human readable (and so needs a hex serialization for readability) or not (in which case a raw byte encoding is appropriate). We'd want to do something similar in this PR as well.