refactor: add Packet::writePackedGuid, remove redundant static methods

Add writePackedGuid() to Packet class for read/write symmetry. Remove
now-redundant UpdateObjectParser::readPackedGuid and
MovementPacket::writePackedGuid static methods. Replace 6 internal
readPackedGuid calls, 9 writePackedGuid calls, and 1 inline 14-line
transport GUID write with Packet method calls.

include/network/packet.hpp

@@ -28,6 +28,7 @@ public:
     uint64_t readUInt64();
     float readFloat();
     uint64_t readPackedGuid();
+    void writePackedGuid(uint64_t guid);
     std::string readString();
 
     uint16_t getOpcode() const { return opcode; }