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

feat: use stackalloc and Span for packet serialization

tests: update, fix tests

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

Pass CanBeNull to string serializer and converter

chore(test): update tests

chore: update tests

chore: move PacketSerializer types to correct namespace

feat(tests): add tests for bool and string converters