Get rid of some unused code.

Looks like some aborted attempt at abstraction, which is no longer used.
Get rid of `special-dots-printing?'.

Asumu said that he doesn't know of any other clients of this parameter
besides my class code.
Add `type-output-sexpr-tweaker'.

Sounds like a bad name, but I don't have anything better.  It's not some
"current-...-print" thing since it works on the sexpr that is intended
to be pretty-printed.  So I went with the above.
Random improvement.

The thing that caught my eye is the missing `#:cache-keys?' which should
really be there.  But not too important.