Modernize the hash module

Modernize the hash module

It seems nowadays libcrypto wants us to

- perform "explicit [algorithm] fetching,"
- cache algorithms to improve performance,
- and stop hardcoding algorithms in the code (which largely doesn't
  really apply in our RFC-bound situation).

https://www.openssl.org/docs/man3.0/man7/crypto.html#ALGORITHM-FETCHING

So fetch explicitely, cache algorithms and try to keep hardcoding at a
minimum.

Also, the code was seemingly unaware that hash size depends on
algorithm, so this was often validated weakly.