Allow saving and resuming from hash state

It would be useful to allow serializing the state of a running hash to be deserialized at a later time and resume adding to the hash state as if no serialization had happened (see this and this).

Perhaps some functions could be added to the public API to allow for de/serializing a running hash. Considering that XXH3_state_s hasn't changed in 6 years, it seems probable that its pretty stable. So perhaps versioning doesn't need to be a concern.