Hash is the SHA hash of Data. NewContext is the new SHA
computation context, while OldContext is the old. OldContext
may be produced by a prior invocation of either sha_new_ctx/3 or
sha_hash_ctx/4 itself.
This predicate allows a SHA function to be computed in chunks,
which may be important while working with Metalink (RFC 5854),
BitTorrent or similar technologies, or simply with big files.