Allow fold et al. to operate on enumerables

[zachallaun]

While trying to fix a spec in data (PR for that incoming), I found that Dialyzer was complaining about this usage of Result.fold/3 – it's expecting a list, but is given a map.

• Update specs of fold/3 – they already handle enumerables
• Refactor fold/2 implementations to work with enumerables
• Update spec for Maybe.justs/ – it already handles enumerables (additionally, refactor without changing behavior)
• Update deps and improve generated documentation by adding links to source (it will assume release tags of the form "MAJOR.MINOR.PATCH", which is the format used in this repo already)
• Update Nix version of Elixir and Erlang

I've confirmed locally that this version of fe fixes the Dialyzer error in data.