Optimized some common subroutines.
Optimized GIVAYF a little.
Clears line link table when clearing screen.
Fixed inserting lines by typing past the right edge.

The screen editor now replicates the behavior of CBM machines
where if you're in the middle of the screen, and type past the
right side of a line, it will insert a line to make room for
more characters, if the resulting logical line is within limits.