Recursive bounds in standard libraries of Erlang and Elixir

[erszcz]

Both Erlang and Elixir use recursive bounds in their standard libraries, which will lead to false positives. Gradualizer overrides such types in its prelude. Should we adopt a similar approach or rather send PRs to Elixir? Or maybe it's possible to lift the "no recursive bounds" restriction in Gradualizer itself?

See #129 for a bit more context.