attn: @zep

A tab character is supposed to move the cursor to the next column that's a multiple of the tab-width setting, but instead it just advances the cursor by tab-width columns every time.

In other words, it currently does this:

function do_tab()
  cursor_x += tab_width
end

And it should do this:

function do_tab()
  cursor_x = (cursor_x + tab_width - 1) % tab_width
end

Oh cool, there is a tab_width setting now!