Add no_std support on nightly

[domenukk]

This PR adds no_std support using the unstable ip_in_core and error_in_core features.

[krisprice]

Thanks @domenukk - I'm seeing a lot more changes here than needs to be. I get the desire to tidy formatting, but doing so makes life harder for people reviewing. Is it possible for you to resubmit this with only the changes to add the no_std support?

[domenukk]

Sorry didn't see this in time. The format was not intended (I guess cargo fmt is muscle memory), may have to do it again from scratch..

[domenukk]

Closing in favor of #51