Cleanup comments and whitespace

We haven't been very consistent about using tabs vs spaces, so use
spaces everywhere. Remove any ugly trailing whitespace. Make sure code
is consistent between chapters. Make sure code is wrapped to within 79
columns because:

  TXT is 80 columns, TXT is 80 columns, TXT is 80 columns.
  Don't trust anything else. Never.
  - Martin Korth, GBATEK <http://nocash.emubase.de/gbatek.htm>