This fixes a problem with the recent clean up in the parsing of type
declarations.  The cleaned up version was correct by H98 (I think), but
unfortunately won't compile the Prelude anymore.

I made the definition a little bit more liberal as follows:
- In `tycl_hdr' allow `gtycon' instead of just `tycon' for the defined type
  constructor.  This allows beyond H98 special syntax like "[]" etc as well as
  tycon ops in parenthesis.  Moreover, it allows qualified names, but the
  renamer flags the latter as errors.
- Allow `gtycon' instead of only `qtycon' for the class name in definitions of
  the form `Eq a => T a' to avoid a whole list of s/r conflicts.  (I have *not*
  checked that the renamer flags misuse, but I would expect that it does.)

ToDo: I think, the renamer should raise an error for all these additional
      forms *unless* -fglasgow-exts is given.

In PrelBase, I needed to replace the infix notation by prefix notation in the
definition of `:+:' and `:*:'.