rewrite rules diagnostics

[Ross Paterson]

(Rewrite rules are very nice, by the way.)

The User's Guide says:

>     * Use -ddump-simpl-stats to see what rules are being fired.
>       If you add -dppr-debug you get a more detailed listing.

but this only happens if the preprocessor symbol DEBUG is defined
in simplCore/SimplMonad.lhs.  Could it be a run-time option, like
the guide says?