Map 'hash' as known api type, add some more tests (#56)

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

src/gmser_api_encoder.erl

@@ -13,7 +13,8 @@
          safe_decode/2,
          byte_size_for_type/1]).
 
--export_type([encoded/0]).
+-export_type([encoded/0,
+             known_type/0]).
 
 -type known_type() :: key_block_hash
                     | micro_block_hash
@@ -38,6 +39,7 @@
                     | native_token
                     | commitment
                     | peer_pubkey
+                    | hash
                     | state
                     | poi
                     | state_trees