Added features and a bug fix from https://github.com/arnaucube/babyjubjub-rs/issues/7

[nanaknihal]

I merged the changes mentioned in https://github.com/arnaucube/babyjubjub-rs/issues/7 for more Point functions, ElGamal encryption/decryption, DLEQ proofs, (unoptimized) serialization/deserialization, a new Fl struct, and a bug fix.

Note: There is one planned change in that issue I did not make: preventing mul_scalar from accepting negative inputs. I imagine this is not unique to mul_scalar, so a better solution would be to replace all instances of BigInt to BigUint except when negative numbers are needed. There seem to be some places negative numbers are needed, and I wanted to be conservative by not changing them. Thus, refactoring BigInt usage to BigUint usage became a bit hairy and I opted not to make that change out of concern with potentially breaking some functionality.

[arnaucube]

Hi, just wanted to comment that I have this PR pending to check, but I'm not finding the needed time slot (as it introduces several new things that need time to be checked), but I'm not forgetting about it. Thanks a lot for the PR :relaxed:

[nanaknihal]

Awesome. Incidentally, I've been working on it a bit today so there is a new push with some pretty minor changes