Proper Golang packaging

This project should be packaged in such a way that it's easy for myself and others to re-use the 65C02 and other components in another project.

Running `godoc` gives some good clues to how everything is packages at this time.

TODO's:

- [ ] Don't export instruction functions (like `CMP`).
- [ ] Refactor where necessary (inline single-use short functions, use proper status getters and setters, etc.)
- [ ] Check organisation of methods across different files