Add Hash_Function::final_truncated()

[reneme]

For certain applications it would actually be nice if Hash_Function could truncate internally. Something like:

void Hash_Function::final_truncated(std::span<uint8_t> out) {
   if(out.size() == output_length()) {
      final(out);
   } else {
      const auto h = final();
      copy_mem(out, std::span{h}.first(out.size()));
   }
}

Or perhaps even pass the shorter span down into the individual hash functions to partially-copy straight from the hash's internal state (if possible).

Originally posted by @reneme in #4455 (comment)