Analogues of `{dropWhile,takeWhile}Antitone` for `IntMap` and `IntSet`

[sjakobi]

Map and Set have these operations, and there is no obvious reason why IntMap and IntSet shouldn't have them too.

If translated directly, the type signature would look like this:

dropWhileAntitone :: (Key -> Bool) -> IntMap a -> IntMap a
takeWhileAntitone :: (Key -> Bool) -> IntMap a -> IntMap a

[sjakobi]

An alternative design could look like this:

data Compare = LessThan | LessThanEqual | GreaterThanEqual | GreaterThan -- | Equal ?

filterCompare :: Compare -> Key -> IntMap a -> IntMap a

The main advantage is that such a function would do the job of both dropWhileAntitone and takeWhileAntitone.

For simplicity and consistency with Map and Set I think direct translations might be best though.

[treeowl]

Consistency is good.

[sjakobi]

For consistency, spanAntitone should be added too.

[meooow25]

I took a shot at implementing this, should I send a PR?

[treeowl]

@meooow25 That would surely be appreciated!