feat: use "in" instead of "ref" in packet serializer

feat: use stackalloc and Span for packet serialization

Merge pull request #47 from Rutherther/feat/pass-nullable

Pass CanBeNull to string serializer and converter

feat(packets): pass nullable to packet serializers

chore: move PacketSerializer types to correct namespace

feat(serializer): fix null representation as stated in #18

feat: split packets definitions into separate assembly