Fix a few style issue in hcheck

This fixes a couple of issue I've seen while fixing the List import:

- removes over-use of printf with putStr/show
- wraps some lines
- removes superfluous parentheses in constructs like 'IO (a)'
- makes sure that in type signatures, the '->' are placed at the
  beginning, not end of the line

We probably need to update the style guide for some of these…

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>