Use `packets.EncodeUint16` in `packets.DecodeShortTopic`?

[dmajda]

The packets.DecodeShortTopic function could use packets.EncodeUint16 internally and just convert the result to a string. However packets.EncodeUint16 is in header.go, which suggests it is intended to be used only in code dealing with packet headers. Maybe move it out of that file and consider it a more general function?