Add code formatter

[wismill]

Following this comment, it would be useful to use Formolu to ensure coding style.

[harendra-kumar]

One hiccup that we saw in adopting fourmolu last time was that there was no way to format like this:

pollCounts
    :: MonadAsync m
    => (a -> Bool)
    -> (Stream m Int -> Stream m Int)
    -> Fold m Int b
    -> Stream m a
    -> Stream m a

It only supported putting the arrows at the end. We have codebase with this style and we have gotten used to it. We did not want to change that. Hopefully it might have support for this now.

[wismill]

There is a PR, but not yet merged.

[adithyaov]

Although not perfect, https://github.com/adithyaov/hindent/tree/composewell-style works for most cases. There are some known cases it does not work for but I manually fix them as it's much easier.

This is in no way a clean way to solve the problem of formatting though.