Add nullable fns to replace option fns

[tapegram]

#120

Update Either and Ior to have nullable alternatives to their option creating methods, other datatypes to follow.

[aballano]

Looks good to me 👍 Thanks @tapegram

[nomisRev]

Awesome @tapegram! Very excited to see a new contributor 🎉

One small comment: we use Ank to verify documentation at build time, and we also use it for post-processing with dokka to turn code snippets in to Kotlin playground snippets :)
We should update the snippets in this PR to also use kotlin:ank:playground.

[nomisRev]

Hey @tapegram,
Sorry, I didn't specify more clearly yesterday and I noticed it's not available in the Ank README.

For a Kotlin playground snippet to be runnable it needs to contain a main function.
It's a bit annoying since it convolutes the snippet, but it's currently the only way we have to make these snippets runnable on the website.

I created a PR to add docs for playground and I added a ticket for Ank to verify the main block in playground 👍

arrow-kt/arrow-ank#53, arrow-kt/arrow-ank#54.

[nomisRev]

Thanks for the contributions @tapegram ❤️