I noticed that the 'hash' serialization type wasn't in the `known_type()` set, although it was supported otherwise.
This PR adds it to the `known_type()` and also exports the type `gmser_api_encoder:known_type()`.
Also, some more tests are added to try to detect this sort of thing in the future.
Co-authored-by: Ulf Wiger <ulf@wiger.net>
Reviewed-on: #56
uwiger