*: Support variable length plumbing.Hash

The variable length for plumbing.Hash is defined at build time, blocked by
tag sha256.

This approach was a trade-off between keeping backwards compatibility while
making progress towards supporting SHA256 with a small amount of changes.

Relates to the SHA256 implementation, defined in #706.

Signed-off-by: Paulo Gomes <pjbgf@linux.com>