API for zip files

[Anaminus]

rbxmk should be able to read and write zip files. Go has a nice API for it, the issue is how the Lua API should look:

• Should be implemented as a format: zips can come from any source.
• The decoded value is non-trivial. How should this be implemented?
• Probably some sort of "Zip" userdata, with methods.
• Should be an interface: Most sources will return a Zip that reads from memory, but file can return a Zip that reads directly from the file.
• This doesn't interact well with the current implementation of formats, which want to read/write an entire []byte all at once.