Use compile time checks for `blake2` hashes (#1083)

* Update blake2 requirement from 0.9 to 0.10

Updates the requirements on [blake2](https://github.com/RustCrypto/hashes) to permit the latest version.
- [Release notes](https://github.com/RustCrypto/hashes/releases)
- [Commits](https://github.com/RustCrypto/hashes/compare/groestl-v0.9.0...blake2-v0.10.0

)

---
updated-dependencies:
- dependency-name: blake2
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

* Use `Blake2Var` instead of `VarBlake2b`

* Move to compile time checks when creating `blake2` hashers

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>