Use GitHub Action to lint Rust

[maximiliankolb]

PR to automatically run "cargo fmt" to verify that PRs do not add code that has not been formatted before.

Example

# positive test on "master":
$ git switch master
$ cargo fmt --all -- --check
$ echo $?
0

# forced non-zero exit if you "break" arbitrary code by hand:
$ cargo fmt --all -- --check
Diff in /home/user/basilk/src/cli.rs:2:

 pub struct Cli;

-impl    Cli{
+impl Cli {
     pub fn read() {
         // If you use `cargo run main.rs`, skip must be 2
         let mut args = env::args().skip(1);
$ echo $?
1