fix(packets): make UpgradeRare packet deserialize correctly

fix(packets): make UpgradeRareSubPacket converter work with "0"

fix(packets): add missing usings

chore: move PacketSerializer types to correct namespace

Merge pull request #32 from Rutherther/combat

Add combat extension

fix(packets): parse 0 UpgradeRareSubPacket as 0, 0

fix(packets): seek when nullable converter finds null.

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

feat: split packets definitions into separate assembly