Context0 is an existing computation context, and Context is the
new context after hashing Data in addition to the previously
hashed data. Context0 may be produced by a prior invocation of
either crypto_context_new/2 or crypto_data_context/3 itself.
This predicate allows a hash to be computed in chunks, which may
be important while working with Metalink (RFC 5854), BitTorrent
or similar technologies, or simply with big files.