* Update `digest` and `blake2` requirement from 0.7 to 0.8

Updates the requirements on [digest](https://github.com/RustCrypto/traits) to permit the latest version.
- [Release notes](https://github.com/RustCrypto/traits/releases)
- [Commits](https://github.com/RustCrypto/traits/compare/digest_v0.7.2...digest-v0.8.1)

Co-authored-by: dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>
Co-authored-by: Pratyush Mishra <pratyushmishra@berkeley.edu>

- contributes to #50,
- depends on #53 and builds on it,
- due to coherence & requirements of `num_traits::{Zero, One}` to implement `std::ops::Add<Self, ..>` and (resp.) `std::ops::Mul<Self, ..>`, I've had to replace the afferent `impl<'a, P: ..> (Add|Mul)<&'a Self> for Group(Affine|Projective)<P>` by direct implementations on `Self`,
- I did not have to fight the borrow checker for this conversion => I think this hints arithmetic operations are called in contexts where the operand is owned,
- hence should this end up on a merge track, we may want to open an issue to convert the `impl<'a, P:..> (Neg|Sub|..)<&'a Self> for ..<P>` trait usage to direct `impl<P:..> (Neg|Sub|..)<Self> for ..<P>`
- the `impl AddAssign for GroupAffine<P>` in curves/models/short_weierstrass_jacobian.rs is provided to fit trait bounds, and without any guarantee of suitability for any particular purpose
- and that, even though I don't think it's used.