Assorted minor Haskell 98 changes:

  * Maximal munch rule for "--" comments
  * _ as lower-case letter, "_" is a reserved id. Prefixing unused
    variable names in patterns with '_' causes the renamer not to
    report such names as being unused.
  * allow empty decls
  * comprehensions are now list comprehensions, not monadic.
  * use Monad.fail to signal pattern matching errors within
    do expressions.
  * remove record punning.
  * empty contexts are now legal  (go wild!)
  * allow records with no fields
  * allow newtypes with a labelled field
  * default default is now (Integer, Double)
  * turn off defaulting mechanism for args & res to a _ccall_.
  * allow LHSs of the form  (a -.- b) x = ...
  * Main.main can now have type (IO a)
  * nuked Void (and its use in the compiler sources.)
  * deriving machinery for Enum now also generate 'succ' and 'pred'
    method bindings.