Upgrade `curv` interface

[survived]

Progress:

• [x] Elliptic curves
• [x] Cryptographic primitives
• [x] Examples

Unresolved questions (subject to move to separate issues):

• Outdated libraries Bunch of libraries need to be updated, including (but not limiting):p256, digest, sha2 See the badge:
• Use curve25519-dalek for ed25519 implementation instead of cryptoxide curve25519-dalek seems to be well-designed and well-documented, it's more actively used than cryptoxide (3 500 000 vs 100 000 total downloads). Besides, we already rely on curve25519-dalek crate to supply ristretto curve
• Hash-to-curve primitive I'd like to add from_uniform_bytes constructor to ECPoint trait that maps uniformly distributed bytes to a point with unknown discrete log. Only ristretto, bls12_381, and ed25519 curves have implementation of hash-to-curve. P256 and K256 require research on this.
• Faster subgroup checks for BLS12-381 Current implementation, in order to validate the point (whether is has order=group_order), it just checks if rP = O (r=group_order). There are new techniques making use of endomorphisms for performing faster subgroup checks

[survived]

@elichai thank you for review! Your comments are very valuable!

on the other hand it makes future constant time operations harder

I think, to support constant time operations, we need underlying library to support points at infinity. E.g. C library that secp256k1 crate is based on, it actually supports points at infinity via dedicated primitives, but secp256k1 crate hides these primitives from their API (for obvious reasons)

[elichai]

I think, to support constant time operations, we need underlying library to support points at infinity.

Yeah I've been thinking about this for a while, See my comment here: https://github.com/ZenGo-X/curv/issues/43#issuecomment-864886342