Generalise `mapM_` for arbitrary `Bifoldable`

[mikeplus64]

Something that works out nicely in uses of streaming is putting chunked results in the functor layer. We can do this with e.g. Of (Vector a) but consumers of this may be tempted to use sequence to get at the results, but (without magic fusion rules) this will necessarily create garbage through short-lived Step constructors for every element of the inner vector/chunk.

What streaming could do instead is generalise functions like Streaming.Prelude.mapM_ to support arbitrary Bifoldables instead of just Of, like:

mapM_ :: Monad m => (a -> m x) -> Stream (Of a) m r -> m r
mapM_ f = loop where
  loop str = case str of
    Return r -> return r
    Effect m -> m >>= loop
    Step (a :> as) -> f a *> loop as

Can be instead:

mapM_ :: (Monad m, Bifoldable of) => (a -> m x) -> Stream (of a) m r -> m r
mapM_ f = loop where
  loop str = case str of
    Return r -> return r
    Effect m -> m >>= loop
    Step step -> bifor_ step f loop

Which would allow the of functor to be anything like data Of a b = !a :< b or data OfChunk a b = OfChunk !(Array a) b and still be able to stream out single results at a time without any extra operations. I think the basically same generalisation can be applied to a lot of the consuming functions like toList, but I haven't looked closely.

Downsides: More compiclated, uglier type signatures.

Alternatives: Just use S.mapM_ (Vector.mapM_ f) and similar.