Enum instances updated to comply with the behaviour that
Haskell 98 specifies. Notable changes,

  * [a..b] is not the empty list when a>b any longer.
  * [x..] and [x,y..] for Enum Int are now bounded lists.

The first change is might be worth bearing in mind when converting
1.4 code to Haskell 98; functions may have made use of the
old behaviour.