`delete`, `alter`, `update`, etc., used a `bin` smart
constructor to avoid installing any non-root `Nil`s. Now only
the ones that could have become `Nil` are checked, which is
a good bit cheaper since they're in cache. `adjustWithKey`
was implemented using `updateWithKey`, but in fact it never
needs to worry about `Nil`s, so implementing it directly
eliminates all such checks.

Make `updateLookupWithKey` in `Data.IntMap.Lazy` strict in its
recursive call to avoid essentially useless lazy pair allocation.