Switch from Travis and AppVeyor to GitHub Actions

[seanmonstar]

AppVeyor is quite slow, and the current script in hyper tests Windows nightly, which can mean Windows testing can be flaky. It'd be nice to move to Travis-CI's new Windows support, running the tests on the same matrix as AppVeyor, but probably sticking with just stable Rust.

[seanmonstar]

I'm noticing this more and more, since AppVeyor won't build any jobs in parallel, nor branches, so if a couple PRs appear, it can take a couple hours to get run (and even just a single PR will run 4 jobs at around ~6-7minutes each, for ~30mins total).

[seanmonstar]

Previous attempt: #1690

[jonas-schievink]

Apparently Azure Pipelines allows up to 10 parallel builds for open source projects, so that might be an alternative. It also supports Windows, macOS and Linux.