Mapping Model v2

[jamierocks]

This pull request contains the work to overhaul Lorenz's mapping model, with some key aims:

1. Increase code reuse through the model.
2. Separate convenience functionality away, to make a clear distinction.
3. Eliminate the necessity for the mapping model to be separate from its own implementation.
4. Clean-up redundant code introduced in previous versions, and not identified.

My current goal is an API similar to the following:

        final MappingSet mappings = new MappingSet();
        final TopLevelClassMapping a = mappings.getOrCreate("a")
                .fields()
                    .getOrCreate("a").setDeobfName("name").getParent()
                    .getOrCreate("b").setDeobfName("age").getParent()
                    .getParent()
                .methods()
                    .getOrCreate("a", "()V").setDeobfName("run").getParent()
                    .getParent();

[jamierocks]

Having discussed this with @Minecrell, this will be renamed to #getParent().