replace argonaut with foreign-generic

[shmish111]

As far as I understand everyone uses foreign-generic rather than argonaut these days. I forked this because

• I want foreign-generic instances
• I don't want a dependency on argonaut

Also use spago and hopefully it can go into the package set one day.

Oh I also formatted with purty without even thinking so the diff has a few whitespace changes, I hope that's ok.

[jvliwanag]

Foreign is different from Argonaut's Json type.

Foreign covers all possible JS types (ex. DOM objects) where Argonaut's Json only covers valid Json constructs (string, number, arrays, objects).

For this reason I still stick to Argonaut and as such, prefer those to still be in place.

[shmish111]

@jvliwanag there is https://pursuit.purescript.org/packages/purescript-foreign-generic/10.0.0/docs/Foreign.Generic#v:decodeJSON etc for dealing with JSON, at least the documentation suggests that everything should be valid JSON. If that is not the case then perhaps these functions should be altered.

Is this possible inconsistency an issue in datetime-iso?