Add Jubjub to ECC stdlib

[alv-around]

Add jubjub to stdlib #1251

Features:

• [x] Proof-Of-Ownership
• [x] VerifyEddsa

[alv-around]

@dark64 @Schaeff. I finished the implementation for the Jubjub curve in the stdlib.

I had to re-arrange some stuff, because many ecc things were written for 254-bit values, and in the jubjub many are 255.

I also wrote in a way so in the future it is easy to integrate the signatures for bls12-377 and other Edwardstwisted curves