Add traverseMonotonic, etc.

[josephcsible]

import Data.Set.Internal

traverseMonotonic :: Applicative t => (a -> t b) -> Set a -> t (Set b)
traverseMonotonic _ Tip = pure Tip
traverseMonotonic f (Bin sz x l r) = flip (Bin sz) <$> traverseMonotonic f l <*> f x <*> traverseMonotonic f r

import Data.Map.Internal

traverseKeysMonotonic :: Applicative t => (k1 -> t k2) -> Map k1 a -> t (Map k2 a)
traverseKeysMonotonic _ Tip = pure Tip
traverseKeysMonotonic f (Bin sz k x l r) = (\l' k' -> Bin sz k' x l') <$> traverseKeysMonotonic f l <*> f k <*> traverseKeysMonotonic f r

bitraverseMonotonic :: Applicative t => (k1 -> t k2) -> (v1 -> t v2) -> Map k1 v1 -> t (Map k2 v2)
bitraverseMonotonic _ _ Tip = pure Tip
bitraverseMonotonic f g (Bin sz k x l r) = (\l' k' x' -> Bin sz k' x' l') <$> bitraverseMonotonic f g l <*> f k <*> g x <*> bitraverseMonotonic f g r

I think we should add these to Data.Set and Data.Map, respectively, as analogues of mapMonotonic, etc. Note that these are unsafe just like they are, and require the exact same care to avoid breaking invariants.

[treeowl]

Would you like to open a PR?

[josephcsible]

If these are indeed good functions to add, then sure. (I just don't want to end up doing all the work of writing tests and documentation for them, and then find out we don't want them.)

[treeowl]

I think they make an awful lot of sense and match the library conventions perfectly. Let's do it.