Basic Rust testing + merge commit detection with Github Actions

[mbestavros]

With Github Actions' general availability date fast approaching (Nov. 13th at time of writing), we figured it'd be a good idea to test the waters by implementing a new check we've been thinking about: detecting merge commits on incoming pull requests.

The test itself is a one-liner, and works by iterating through all commits new on a pull request and checking if they have multiple parents (a characteristic of merge commits). If it finds any, it fails; if not, it succeeds.

This test is implemented inside a shiny new Github Actions testing config! I've included a few basic Rust tests (build, test, formatting) and arranged them to demonstrate how tests might be organized with GH Actions. There's some nice integration throughout the Github UI; it'll be interesting to see how well it fares once it hits GA.

Resolves #47.

[mbestavros]

We've learned from this pull request that GH Actions will not actually run new tests added by pull requests from forked repositories, which makes it rather useless for us as a CI solution. I'm closing this PR and will be porting over the new test into Travis.