Reorganise code structure

[Palmr]

Currently almost everything is public, which isn't great.

The only way to parse your own byte array requires calling the parsing code yourself and dealing with the result from nom, which leaks parsing implementation details outside of the crate.

There should probably be some more top level functions in lib for parsing from paths, jars, byte arrays etc.
Structs for the public data types should be together rather than scattered about.

Parsers should be together and non-public.