New curve interfaces

[annenkov]

Purpose

Make the Curve trait and related functionality (e.g. finine field interfaces) better suitable for adding new curve implementations. Add the curve25519 implementation using the updated interface. Add benchmarks of the curve25519 curve with our bulletproof implementation.

Changes

• add Field and PrimeField traits; the idea is that they contain only what we need and don't have extra dependencies; moreover, we can provide implementation for external type, which was not possible with ff::Field;
• make it possible to override the generic multiexp algoritm implementation, if a certain curve provides an optimised version;
• implement Field, PrimeField for the BLS12-381 curve (including a blanket inplementation of Field for any F : ff::Field;
• implement Field, PrimeField and Curve for The ristretto representation of curve25519-dalek;
• add benckmarks:
  • multiexp algorithms from curve25519-dalek exposed through our interface (authored by @hamiidreza);
  • range proof performance curve25519;
  • range proof from dalek's bulletproofs (authored by @hamiidreza). (was exluded in the development process)

Checklist

• My code follows the style of this project.
• The code compiles without warnings.
• I have performed a self-review of the changes.
• I have documented my code, in particular the intent of the
  hard-to-understand areas.
• I have updated the CHANGELOG.

[abizjak]

I thought the idea was to have a minimal PR that only does the introduction of new abstractions. This seems to have more, e.g., why is there ark-ff as a new dependency?

[hamiidreza]

Added some minor comments and suggestions. Otherwise, looks good to me.