Add nightly tests to github workflows

[coreylowman]

The command to run locally is cargo +nightly test --tests. There is probably a way to translate this into the cargo github workflow command, I vaguely remember it being documented

[JYudelson1]

I checked https://github.com/actions-rs/meta, and it seems to just be another matrix in the .yml file to run on both stable and nightly. Made a PR for it at #294

[coreylowman]

Thanks a bunch!