This change provided by Alastair Reid is a bunch of wibbles which fix
some severe performance problems in the copy of the Pretty library
distributed with Hugs-Sept99.

The problems show up when making heavy use of hsep (eg printing large
numbers of comma separated lists which tend to run over the end of
line).  The problems manifest themselves as the infamous "control
stack overflow" and seem to be due to the generation of large
Int thunks that look something like this:

  80 - 4 - 1 - 1 - 3 - 1 - ... -1

(There may be a few +'s in there too but -'s predominate.)