Unzip Map and IntMap more strictly

[treeowl]

The previous Map (same for IntMap throughout) instance
would first map eagerly over tha Map, producing an entire
Map full of thunks to apply the unzipWith function. Then
it would build two more entire Maps full of thunks to select
components of each pair. We can do it eagerly using
Data.Biapplicative.traverseBia and a simple pair bifunctor.

[treeowl]

Note: if you really want to be super-lazy, we can at least avoid three entire Maps and needing to worry about whether the resulting Maps even contain selector thunks (or if they can leak space).

unzipWith f = unSBPair . traverseBia (SBPair . blah)
  where
    blah c = let
      {-# NOINLINE fc #-} -- make sure the result of f c is shared
      {-# NOINLINE a #-} -- make sure we get selector thunks
      {-# NOINLINE b #-}
      fc = f c
      (a, b) = fc
    in (a, b)

[treeowl]

I've opened #164 with a version that preserves the precise laziness of the current version but without the leak potential and without building a third map structure.

[treeowl]

(Personally, I like this version better than that one, but it's not my package.)