Clean up comments
Fix comment indentation
Save 53 bytes by removing the hidden message
Clarify use of Y register in calc_scrline
Replace screen line pointer calculation with a fast table lookup
Use table lookup to get last line instead of conditionals
Optimize register usage in screen line pointer routines
Save 80 bytes by removing filler bytes in the tab stop buffer
The tab stop buffer is filled during init.  Removing these bytes
saves disk space without affecting the program.
Replace character translation with a fast table lookup