* unboxed the state in ST and IO, and specialised IO (it is no longer built
  on top of ST).

* reversed the sense of -fwarn-{overlapped,incomplete}-patterns.  The driver
  now has to have the flag to provide the warning.

* added the -Wnot, -W, and -Wall options.  Set up a default set of warnings,
  namely -fwarn-{overlapped,incomplete}-patterns.