Fuse Map.map with foldrFB and foldlFB

We have a rule

``` haskell
{-# RULES "Map.toAscList" [~1] forall m . toAscList m = build (\c n -> foldrFB (\k x xs -> c (k,x) xs) n m) #-}
```

and a similar one for `toDescList`. I think we probably want to add rules like

``` haskell
{-# RULES "Map.foldrFB/map" forall (m :: Map k v) (f :: Int -> v' -> r -> r) (g :: v -> v') (n :: r) .
foldrFB f n (map g m) = foldrFB (\i x r -> f i (g x) r) n m #-}
{-# RULES "Map.foldrFB/mapWithKey" forall (m :: Map k v) (f :: Int -> v' -> r -> r) (g :: Int -> v -> v') (n :: r) .
foldrFB f n (mapWithKey g m) = foldrFB (\i x r -> f i (g i x) r) n m #-}
```

and similarly for `foldlFB`. These will ensure, for example, that mapping over a `Map` and then converting the result to a list will not produce intermediate structures.