`gnark/std` needs some love

[gbotrel]

gnark/std started as ad-hoc proof of concepts to validate gnark design choices and use cases like rollups.

Needs a re-visit to ensure the packages under gnark/std:

• are rock solid
• have coherent APIs names and patterns
• are well documented

[SherLzp]

Already make a PR for Neg of point in std/twistededwards/point

--------- below is fixed----------------- Hi, please see #103

[gbotrel]

Comment; gnark/std is tested using Groth16 on some curves only.

Need to use backend.Implemented() and ecc.Implemented() to ensure good test coverage across backends en curves.