File Encoders for all file formats in `d2common/d2fileformats/

[gravestench]

We currently only have file decoders in d2fileformats. Most of the Load functions take a byte slice as an argument, and parse the bytes into a struct for that file type. We need to be able to convert a struct instance back to a byte slice.

[rjnienaber]

I did a cursory check and it seems like d2dcc and d2mpq are the only file formats that still require a .Marshal method. Is this required for those formats? If so, I'd like to take a shot at the .dcc one but I don't have an example file that I could commit to the repo.