CI: Maintenance

[EwoutH]

A bit of maintenance to the CI configuration:

• Run on pull requests and once a month (to test new Rust versions)
• Add build matrix to add runs ons Windows and macOS
• Update to actions/checkout v2 (faster and maintained)
• Always install latest Rust version with actions-rs/toolchain
• Use cargo build --release to test optimized release build

[EwoutH]

@hugoledoux Hi! Could you review this PR?

[hugoledoux]

cheers