Adds automatic tests and checks on every commit / PR via Github Actions

[phayes]

This PR does the following using github actions:

1. Runs cargo audit every night
2. When a point-release is tagged, automatically uploads it to crates.io (this may not be your preferred workflow, let me know and I can remove this).
3. On every commit / PR, does the following:
   A. Runs cargo test
   B. Runs cargo fmt -- --check to check that code formatting adheres to rustfmt standards
   C. Runs cargo clippy to check that things conform to clippy's lints
   D. Generates test coverage statistics and uploads it to codecov.io

For item 2, it will need CARGO_API_KEY secret to be in place, for item 3D, it will need CODECOV_TOKEN secret to be in place.