`packet_types`

[mixmix]

Type 0 = fixed length = payload is EXACTLY 48 bytes
Type 1 = variable length = payload could be <= "size" and be just on main chain, OR may be > "size" and spill over into side-chain of chunks

[mixmix]

note for Type 1 has 2 modes
a) length + content + padding fits in 48 bytes (NOTE this misses the hash pointer field)
b) length + content_first_chunk + hash_content_next_chunk

[mixmix]

TODO - test vectors for these 2 cases