nn_boss: Large rework with various improvements

Lots of internal changes. On the surface this only fixes a crash in Mario & Sonic Rio 2016 (at least what I saw from my testing) but it may affect more games.

Summary of changes:
- Rewrite code to use newer cafeExportRegisterFunc
- Simplify code by merging namespaces and structs of the same types
- Correctly set ppc vtables for the virtual boss classes
- Fix some wrong function definitions and implement a little bit more of the boss API (mainly constructors and destructors)