Drop unused `either` dependency

It seems like the either trans. instances were replaced (#39) a long time ago and at this moment there is no any usage of either package at all
I was able to compile the latest version of this package with cabal either dep. dropped
Please check out if the dependency can be dropped

[acowley]

Thank you! If you open a PR with that change, I will gladly merge it.

I'd really like to get us over to GH Actions for CI to better automate testing changes like this, but haven't had the chance to do so.

[gelisam]

I'd really like to get us over to GH Actions for CI to better automate testing changes like this

I'm giving it a try in #61