Made many changes.
- Changed affix_stack to comply with the new design so it carries
  the root AFFIX component.
- Changed baseaffix so an AFFIX no longer refers to a stack. Instead
  an AFFIX just points to the next AFFIX in the stack.
- Added the functionalities pop(), peek() and push() into baseaffix
  so the AFFIXs themselves can modify the stack below them.