Version 3 tables.

This adds support for yet another hash function: sha256. We keep the
Keccak Hash as version #2, because it works well and has been around
for years in an experimental branch of this repo. As for Keccak,
we only use 20 bytes of the sha256 hash, which is safe enough and
helps to keep the patch as small as possible.

The sha256 implementation was taken from git v2.25.1.