[m-dev.] trivial diff: disable deforestation

[Simon Taylor]

> > compiler/options.m:
> > 	Disable deforestation due to a bug which has something to do with
> > 	quantification and mode information not being kept consistent. 
> 
> Could you elaborate about what conditions trigger the bug?
> e.g. do you have a test case?

It fell over on the compiler (prog_io.m?). The problem was triggered by
stopping the removal of goals (especially unifications) with no outputs -
this caused aborts in common.m for constructions where the constructed
variable was not a non-local of the goal.

Simon.